Support Windows UNC paths in loaders

[KapJI]

Replace os.path.normpath with str(pathlib.Path()) which correctly fixes path separators when UNC path is provided.

Tested that this code doesn't escape search dir, e.g.:

>>> str(pathlib.Path(posixpath.join("C:\\abc", "e:d\\ef")))
'C:\\abc\\e:d\\ef'
>>> str(pathlib.Path(posixpath.join("\\\\?\\C:\\abc", "e:d\\ef")))
'\\\\?\\C:\\abc\\e:d\\ef'

• fixes #1675

Checklist:

• [ ] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [ ] Add or update relevant docs, in the docs folder and in code.
• [ ] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [ ] Add .. versionchanged:: entries in any relevant code docs.
• [x] Run pre-commit hooks and fix any issues.
• [x] Run pytest and tox, no tests failed.