Only depend on importlib-metadata under Python 3.10

[pawamoy]

Fixes #2958

We always try to import importlib_metadata first.
I can switch to a condition on the current Python version, instead of a try/except, if you think that's more robust.

Without the type: ignore comments I was getting this:

mkdocs/utils/__init__.py:30: error: Incompatible import of "entry_points" (imported name has type "Callable[[], Dict[str, List[EntryPoint]]]", local name has type "Callable[[KwArg(Any)], Union[EntryPoints, SelectableGroups]]")  [misc]
mkdocs/plugins.py:22: error: Incompatible import of "entry_points" (imported name has type "Callable[[], Dict[str, List[EntryPoint]]]", local name has type "Callable[[KwArg(Any)], Union[EntryPoints, SelectableGroups]]")  [misc]
Found 2 errors in 2 files (checked 58 source files)

EDIT: might need to squash to reword the commit: below instead of under (pardon my french).

[oprypin]

Thanks for the research.

Usually the try-except imports apply nicely to stdlib modules that have 3rd party shims and you want to prioritize stdlib if it exists. And you do have to try stdlib first. The reason is that otherwise the behavior will be affected by the external state of whether this package is installed: Confusing error if it's not installed when it should be and just arbitrarily using the (hopefully) equivalent implementation when it happens to be installed but was not requested.

Here, the stdlib module existing is not enough, so that approach can't be used. So I think we indeed have to use an explicit version check.

if sys.version_info >= (3, 10):
    from importlib.metadata import entry_points
else:
    from importlib_metadata import entry_points  # type: ignore

[pawamoy]

Thanks, I've applied your suggestion.

[oprypin]

Thanks, this is good.

And yes, I'm seeing (3.10 locally and 3.9 in CI) that now apparently # type: ignore is not needed.