text: Title: 3.3.1: pytest hangs in `tests/test_compat.py::TestMaybeAsync::test_cancel_scope[trio]` unit
  Body: `trio` 41.0.
  ```console
  + /usr/bin/pytest -ra -p no:itsdangerous -p no:randomly -v
  =========================================================================== test session starts ============================================================================
  platform linux -- Python 3.8.12, pytest-6.2.5, py-1.10.0, pluggy-0.13.1 -- /usr/bin/python3
  cachedir: .pytest_cache
  benchmark: 3.4.1 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
  hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/home/tkloczko/rpmbuild/BUILD/anyio-3.3.1/.hypothesis/examples')
  rootdir: /home/tkloczko/rpmbuild/BUILD/anyio-3.3.1, configfile: pyproject.toml, testpaths: tests
  plugins: anyio-3.3.1, forked-1.3.0, shutil-1.7.0, virtualenv-1.7.0, expect-1.1.0, flake8-1.0.7, timeout-1.4.2, betamax-0.8.1, freezegun-0.4.2, aspectlib-1.5.2, toolbox-0.5, rerunfailures-9.1.1, requests-mock-1.9.3, cov-2.12.1, flaky-3.7.0, benchmark-3.4.1, xdist-2.3.0, pylama-7.7.1, datadir-1.3.1, regressions-2.2.0, cases-3.6.3, xprocess-0.18.1, black-0.3.12, asyncio-0.15.1, subtests-0.5.0, isort-2.0.0, hypothesis-6.14.6, mock-3.6.1, profiling-1.7.0, Faker-8.12.1, nose2pytest-1.0.8, pyfakefs-4.5.1, tornado-0.8.1, twisted-1.13.3, aiohttp-0.3.0
  collected 1245 items
  
  tests/test_compat.py::TestMaybeAsync::test_cancel_scope[asyncio] PASSED                                                                                              [  0%]
  tests/test_compat.py::TestMaybeAsync::test_cancel_scope[asyncio+uvloop] PASSED                                                                                       [  0%]
  tests/test_compat.py::TestMaybeAsync::test_cancel_scope[trio]
  ```
  .. and that is all.
  `ps auxwf` shows only
  ```console
  tkloczko 2198904  1.4  0.0 6576524 115972 pts/7  S+   10:35   0:10                  \_ /usr/bin/python3 /usr/bin/pytest -ra -p no:itsdangerous -p no:randomly -v
  ```
  ```console
  [tkloczko@barrel SPECS]$ strace -p 2200436
  strace: Process 2200436 attached
  futex(0x55a89bdec820, FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME, 0, NULL, FUTEX_BITSET_MATCH_ANY
  ```