Fix: tolerate a concurrent removal of a dead symlink at session finish

[zyv]

I run many pytest sessions in parallel (mutmut forks a worker per CPU, and each worker runs pytest.main). Now and then a session fails at session finish:

  File ".../_pytest/pathlib.py", line 371, in cleanup_numbered_dir
    cleanup_dead_symlinks(root)
  File ".../_pytest/pathlib.py", line 357, in cleanup_dead_symlinks
    left_dir.unlink()
FileNotFoundError: [Errno 2] No such file or directory: '.../pytest-of-<user>/pytest-current'

cleanup_dead_symlinks checks that a link is dead, then unlinks it, and another session can unlink the same link in-between. This reproduces for me on macOS with pytest 9.1.1 (it takes a few rounds):

printf 'def test(tmp_path):\n    pass\n' > test_a.py
export PYTEST_DEBUG_TEMPROOT=$(mktemp -d)
for round in $(seq 10); do
  for i in $(seq 32); do
    python -m pytest -q -p no:cacheprovider -o tmp_path_retention_count=1 test_a.py > out.$i 2>&1 &
  done
  wait
  grep -l FileNotFoundError out.*
done

The fix passes missing_ok=True to unlink.

I haven't found any already reported issues to that effect.

The problem was identified using a supervised agent, and I have reviewed the code and the test - mutmut does no longer attribute flaky mutant kills to pytest session crashes.

[zyv]

I have added the changelog entry under this PR's number, and the last edit is cosmetic - it hopefully removes 1 uncovered line in the mock. I'm not sure what was up with readthedocs; I guess it was some problem unrelated to my changes, but everything seems to be green now.