Ensure urljoin behaviour is correct

[davfsa]

Upstream bugfix for mkdocstrings/mkdocstrings#443

I have attempted to write the necessary tests but the layout of the library is interfering a bit:

from mkdocs_gen_files import plugin

Traceback (most recent call last):
  File "/home/davfsa/coding/mkdocs-gen-files/tests/test_plugin.py", line 4, in <module>
    from mkdocs_gen_files import plugin
  File "<frozen importlib._bootstrap>", line 1075, in _handle_fromlist
  File "/home/davfsa/coding/mkdocs-gen-files/mkdocs_gen_files/__init__.py", line 20, in __getattr__
    return getattr(FilesEditor.current(), name)
  File "/home/davfsa/coding/mkdocs-gen-files/mkdocs_gen_files/editor.py", line 94, in current
    config = load_config("mkdocs.yml")
  File "/home/davfsa/coding/mkdocs-gen-files/.venv/lib/python3.10/site-packages/mkdocs/config/base.py", line 236, in load_config
    raise exceptions.Abort(
mkdocs.exceptions.Abort: Aborted with 3 Configuration Errors!

Not sure what the best way to fix this would be

[oprypin]

Thanks!

[oprypin]

I finally made a release with this. https://github.com/oprypin/mkdocs-gen-files/releases/tag/v0.3.5