Enforce return types for typed public functions & mark pkg_resources as py.typed

[Avasam]

Summary of changes

Merge #4390 and #4391 first !

Addresses #4397 (comment)

This also marks pkg_resources as py.typed as it is now more on par (or even improved) over typeshed's stubs. With the exception of extern imports causing a loss of typing, but that's a completely different discussion and pkg_resources as a deprecated module, isn't affected too badly. #4457

Step 4 & 5 of #2345 (comment)

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)

[Avasam]

@abravalheri @jaraco out of all my currently open PRs, this is the one I'd like to prioritise most, as it marks pkg_resources as py.typed, launching the 6 months countdown to remove pkg_resources from typeshed.

I've also been thinking of maybe not enforcing ANN2 on the setuptools side yet, since I'd probably disable it later to progressively type setuptools like I did pkg_resources

[jaraco]

LGTM. I didn't see any changes in the implementation. Please confirm that's the case.

The only risk with merging this is if we have to back out #4457, probably some of the type checking will need to back off, but that outcome seems unlikely at this point.

[Avasam]

I didn't see any changes in the implementation. Please confirm that's the case.

I re-reviewed the PR and I don't see any change in implementation either.

if we have to back out #4457, probably some of the type checking will need to back off, but that outcome seems unlikely at this point.

Even that I feel would be minor.