🐛 Fix ImportError that could occurs on older Python 3.5.x

[Ousret]

First discovered by ros-infrastructure/ros_buildfarm#905 Not all Python 3.5.x are concerned. Latest minor is OK (3.5.10).

Was very unexpected. This PR should fix that promptly.

[Ousret]

In a attempt to "backport" os.PathLike on Python 3.5 and later on comply with flake8, I accidentally triggered a miscellaneous eval on the os package.

try:
    from os import PathLike
except ImportError:  # pragma: no cover
    import os  # was added to comply with flake8 and triggered the expr eval in Union[str, "os.PathLike[str]"].
    PathLike = Union[str, "os.PathLike[str]"]  # type: ignore