Fix test_dir_without_exec_permission on Python 3.14

[cjwatson]

One unit test fails on Python 3.14 as follows:

_____________ TestFolderTraversal.test_dir_without_exec_permission _____________

self = <test.unit.scan.test_folder_traversal.TestFolderTraversal object at 0x7f5530606350>
tmp_path = PosixPath('/tmp/pytest-of-sbuild/pytest-0/test_dir_without_exec_permissi0')
fs_perm_tool = <test.unit.conftest.UnixPermTool object at 0x7f55301ca510>

    @pytest.mark.skipif(
        platform.system() == 'Windows',
        reason='Unix-only filesystem permissions are tested',
    )
    def test_dir_without_exec_permission(self, tmp_path, fs_perm_tool):
        """Test that a excluded directory/file without permissions emits warnings."""
        no_perm_dir = tmp_path / 'no_perm_dir'
        no_perm_dir.mkdir()
        (no_perm_dir / 'file.txt').touch()
        (no_perm_dir / 'file2.txt').touch()
        # chmod -x no_perm_dir
        no_perm_dir.chmod(0o600)

        scan_policy = ScanPoliciesManager()
        reporter = ProgressReport(sys.stdout, False)

        folder = LocalFolder(str(tmp_path))
        local_paths = folder.all_files(reporter=reporter, policies_manager=scan_policy)
        absolute_paths = [path.absolute_path for path in local_paths]
        assert not absolute_paths

        # Check that no access warnings are issued for the excluded directory/file
>       assert set(reporter.warnings) == {
            f'WARNING: {tmp_path/"no_perm_dir/file.txt"} could not be accessed (no permissions to read?)',
            f'WARNING: {tmp_path/"no_perm_dir/file2.txt"} could not be accessed (no permissions to read?)',
        }
E       AssertionError: assert {'WARNING: /t...en symlink?)'} == {'WARNING: /t...ns to read?)'}
E
E         Extra items in the left set:
E         'WARNING: /tmp/pytest-of-sbuild/pytest-0/test_dir_without_exec_permissi0/no_perm_dir/file2.txt could not be accessed (broken symlink?)'
E         'WARNING: /tmp/pytest-of-sbuild/pytest-0/test_dir_without_exec_permissi0/no_perm_dir/file.txt could not be accessed (broken symlink?)'
E         Extra items in the right set:
E         'WARNING: /tmp/pytest-of-sbuild/pytest-0/test_dir_without_exec_permissi0/no_perm_dir/file.txt could not be accessed (no permissions to read?)'
E         'WARNING: /tmp/pytest-of-sbuild/pytest-0/test_dir_without_exec_permissi0/no_perm_dir/file2.txt could not be accessed (no permissions to read?)'
E         Use -v to get more diff

test/unit/scan/test_folder_traversal.py:721: AssertionError

This is due to python/cpython#118243. I'm not sure how important it is to preserve the exact same warning message here, but the conservative approach is to do so;
https://docs.python.org/3/library/pathlib.html advises using Path.stat to distinguish these cases.