text: Title: "Too many open files"
  Body: ## Issue
  
  Running tox in https://github.com/nedbat/django_coverage_plugin results in "Too many open files" errors from inside tox, virtualenv, etc.
  
  ## Environment
  
  Provide at least:
  
  - OS: Mac OS 15.2
  
  <details open>
  <summary>Output of <code>pip list</code> of the host Python, where <code>tox</code> is installed</summary>
  
  ```console
  % pip list
  Package            Version
  ------------------ -----------
  attrs              25.1.0
  backports.tarfile  1.2.0
  build              1.2.2.post1
  cachetools         5.5.1
  certifi            2024.12.14
  chardet            5.2.0
  charset-normalizer 3.4.1
  click              8.1.8
  click-log          0.4.0
  colorama           0.4.6
  distlib            0.3.9
  docutils           0.21.2
  filelock           3.17.0
  id                 1.5.0
  idna               3.10
  importlib_metadata 8.6.1
  jaraco.classes     3.4.0
  jaraco.context     6.0.1
  jaraco.functools   4.1.0
  Jinja2             3.1.5
  keyring            25.6.0
  markdown-it-py     3.0.0
  MarkupSafe         3.0.2
  mdurl              0.1.2
  more-itertools     10.6.0
  nh3                0.2.20
  packaging          24.2
  pip                24.2
  platformdirs       4.3.6
  pluggy             1.5.0
  Pygments           2.19.1
  pyproject-api      1.9.0
  pyproject_hooks    1.2.0
  readme_renderer    44.0
  requests           2.32.3
  requests-toolbelt  1.0.0
  rfc3986            2.0.0
  rich               13.9.4
  scriv              1.5.1
  setuptools         74.0.0
  tomli              2.2.1
  tox                4.24.1
  twine              6.1.0
  typing_extensions  4.12.2
  urllib3            2.3.0
  virtualenv         20.29.1
  wheel              0.44.0
  zipp               3.21.0
  ```
  
  </details>
  
  ## Output of running tox
  
  <details open>
  <summary>Output of <code>tox -rvv</code></summary>
  
  It's 5200 lines, I put it in a gist: https://gist.github.com/nedbat/9f54a1756f4d3d3f34d00cf9cceb8440
  
  </details>
  
  ## Minimal example
  
  <!--  If possible, provide a minimal reproducer for the issue. -->
  
  ```console
  % git clone https://github.com/nedbat/django_coverage_plugin.git
  % cd django_coverage_plugin
  % git checkout a1f1d93bccbd495e0d019052a6e411f88f5cbc6f
  % pip install -r requirements.txt
  % tox 
  ```
  
  Sorry I don't have a smaller example.