Permission errors are misreported on Python 3.14

[arecarn]

On Python 3.14, stowing into a tree whose source directory has had its execute permission removed reports the wrong error. Expected PermissionDenied, actual ConflictsWithExistingLink.

Cause

pathlib.Path.exists() no longer propagates PermissionError. Same directory, execute permission removed:

python 3.14.4
  Path(src/aaa).exists() = False
  Path(src/aaa).is_dir()  = False
  iterdir() raises PermissionError

python 3.13.0
  Path(src/aaa).exists() raises PermissionError

AbstractBaseStow._collect_actions calls dest_path.exists() inside a try/except PermissionError and depends on that raise to add a PermissionDenied error. On 3.14 the call returns False, the permission problem goes unnoticed, and collection continues to a misleading conflict error.

Impact

The CI matrix is 3.10-3.13, where the behaviour is correct, so this does not currently show up in CI. It will as soon as 3.14 is added to the matrix.

Two tests cover the affected paths and are marked xfail on 3.14 pointing at this issue:

• tests/test_stow.py::test_stow_unfolding_with_first_sources_execute_permission_removed
• tests/test_unstow.py::test_unstow_folding_with_multiple_sources_with_execute_permission_unset

Fix direction

Stop inferring permission failures from exists(). Check access explicitly (utils.is_directory_executable already exists) or move the detection to the point where the directory is actually read, which still raises.