replace pkg_resources with importlib.metadata

[Panaetius]

Fixes #952

Proposed Changes

• replaces pkg_resources.iter_entry_points with importlib.metdata.entry_points
• Conditionally installs importlib_metadata backport depending on python version

One thing to note, the if hasattr(all_entry_points, "select"): check is slightly awkward. In python 3.8 and 3.9, entry_points() returns a plain dict, but in python 3.10 and the backport (on 3.7) it returns a SelectableGroups objects that inherits from dict. Using select() on that object is slightly faster (~55ms) than using it through the dict interface. Since it inherits from dict, isinstance(x, dict) wouldn't work, and since SelectableGroups is not available in the 3.8/3.9 version, isinstance(x, SelectableGroups) wouldn't work either. hence the check via hasattr. This makes it perform equally fast across python versions.

[Panaetius]

I'm not sure why it fails on python 3.7 with

error: Module "importlib.metadata" has no attribute "entry_points"  [attr-defined]

on 3.7 it should be

ModuleNotFoundError: No module named 'importlib.metadata'

which is what I get when testing locally.

[nicholascar]

Hmmm... I'll try and run locally tomorrow

[aucampia]

I think the best option is to add this to compat and use sys.version_info so mypy can ignore it. I will try make a PR tomorrow morning to fix this.

Don't hesitate pinging me for mypy errors.

[aucampia]

@Panaetius I created a PR against your fork to fix this: Panaetius#1

[aucampia]

For more info on what is going wrong here: https://mypy.readthedocs.io/en/stable/common_issues.html#python-version-and-system-platform-checks

[Panaetius]

I merged your PR, thanks for looking into this!

[nicholascar]

Thanks to both of you @Panaetius and @aucampia!