text: Title: tox hangs but no subprocess running anymore
  Body: ## Issue
  
  Some times (not reproducable every time but i run into the issue quite often) tox does not terminate.
  I run it with
  ```console
  python3 -m tox run-parallel --conf "${config_dir}"/tox.ini --parallel auto --parallel-no-spinner --quiet
  ```
  This command will then never terminate, but with htop it seems none of the external tools is running anymore:
  ![image](https://github.com/tox-dev/tox/assets/70687692/f07241dc-b2f8-407b-9e41-14188e14aea1)
  
  When i add `--exit-and-dump-after 10` it does terminate and will produce the below output.
  
  ## Environment
  
  Provide at least:
  
  - OS: Ubuntu 20.04
  - Python: 3.8.10
  
  <details open>
  <summary>Output when terminated using exit-and-dump-after...</summary>
  
  ```console
  Timeout (0:00:10)!
  Thread 0x00007f87e2ffd700 (most recent call first):
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/execute/local_sub_process/read_via_thread_unix.py", line 34 in _read_available
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/execute/local_sub_process/read_via_thread_unix.py", line 23 in _read_stream
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f87f9ffb700 (most recent call first):
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/execute/local_sub_process/read_via_thread_unix.py", line 34 in _read_available
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/execute/local_sub_process/read_via_thread_unix.py", line 23 in _read_stream
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f887dffb700 (most recent call first):
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 78 in _worker
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f887e7fc700 (most recent call first):
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 78 in _worker
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f887effd700 (most recent call first):
    File "/usr/lib/python3.8/subprocess.py", line 1764 in _try_wait
    File "/usr/lib/python3.8/subprocess.py", line 1806 in _wait
    File "/usr/lib/python3.8/subprocess.py", line 1083 in wait
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/execute/local_sub_process/__init__.py", line 100 in wait
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/tox_env/api.py", line 388 in execute
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/session/cmd/run/single.py", line 106 in run_command_set
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/session/cmd/run/single.py", line 85 in run_commands
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/session/cmd/run/single.py", line 48 in _evaluate
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/session/cmd/run/single.py", line 36 in run_one
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/session/cmd/run/common.py", line 322 in _run
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 57 in run
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 80 in _worker
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f887f7fe700 (most recent call first):
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 78 in _worker
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f887ffff700 (most recent call first):
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 78 in _worker
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f889cf27700 (most recent call first):
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 78 in _worker
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f889d728700 (most recent call first):
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 78 in _worker
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f889df69700 (most recent call first):
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 78 in _worker
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f889e76a700 (most recent call first):
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 78 in _worker
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f889ef6b700 (most recent call first):
    File "/usr/lib/python3.8/concurrent/futures/thread.py", line 78 in _worker
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f889f76c700 (most recent call first):
    File "/usr/lib/python3.8/threading.py", line 306 in wait
    File "/usr/lib/python3.8/threading.py", line 558 in wait
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/util/spinner.py", line 87 in render
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f889ff6d700 (most recent call first):
    File "/usr/lib/python3.8/threading.py", line 302 in wait
    File "/usr/lib/python3.8/threading.py", line 558 in wait
    File "/usr/lib/python3.8/concurrent/futures/_base.py", line 244 in as_completed
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/session/cmd/run/common.py", line 346 in _queue_and_wait
    File "/usr/lib/python3.8/threading.py", line 870 in run
    File "/usr/lib/python3.8/threading.py", line 932 in _bootstrap_inner
    File "/usr/lib/python3.8/threading.py", line 890 in _bootstrap
  
  Thread 0x00007f88a1e38740 (most recent call first):
    File "/usr/lib/python3.8/threading.py", line 1027 in _wait_for_tstate_lock
    File "/usr/lib/python3.8/threading.py", line 1011 in join
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/session/cmd/run/common.py", line 251 in execute
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/session/cmd/run/parallel.py", line 85 in run_parallel
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/run.py", line 46 in main
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/run.py", line 20 in run
    File "/myhome.../.venv/lib/python3.8/site-packages/tox/__main__.py", line 6 in <module>
    File "/usr/lib/python3.8/runpy.py", line 87 in _run_code
    File "/usr/lib/python3.8/runpy.py", line 194 in _run_module_as_main
  
  ```
  
  </details>
  
  ## Output of running tox
  
  <details open>
  <summary>Tox output is simply incomplete:</summary>
  
  ![image](https://github.com/tox-dev/tox/assets/70687692/1f11e33e-4a0e-4cf2-aa4c-73694ee029e7)
  (special characters at the end accidentally added)
  </details>
  
  ## Minimal example
  
  Not sure how to reproduce currently.
  