pkg_resources: fix incorrect implicit return types

[Avasam]

Summary of changes

Checked all public methods (def ([^_]|__)) in pkg_resources for their implied return type using pyright/pylance. Updated annotations for those that seemed incorrect or returned a partial Any even after #4390 .

Step 3.3 of #2345 (comment)

Once this #4355 and #4390 are merged, pkg_setuptools will be on par with Typeshed

Pull Request Checklist

• Changes have tests (type-checking and existing basic tests for syntax errors)
• News fragment added in newsfragments/. (no user facing changes yet until pkg_resources is considered typed)
  (See documentation for details)

[abravalheri]

Thank you very much @Avasam.

Other than the open question of what to do with extern, I think all the other changes are good.

[abravalheri]

I think we should defer any changes like this until we have a decision about how to reconcile extern and type-cheking.

if TYPE_CHECKING:
    # ...
    def yield_lines(iterable: "_NestedStr") -> chain[str]: ...

    # Trick type-checkers into seeing the original instances for type comparisons
    from packaging import version as _packaging_version
    from packaging import requirements as _packaging_requirements

[Avasam]

Even if we did, this part would be useful. At least the two imports.

Even if static type-checkers understood the extern imports, they would be treated as a different type than the originals:

from ._vendor.packaging.version import Version as parse_version
from packaging.version import Version

def foo(bar: Version): ...
foo(parse_version("0.0.0"))  # type error

Pyright:

error: Argument of type "Version" cannot be assigned to parameter "bar" of type "Version" in function "foo"
    "pkg_resources._vendor.packaging.version.Version" is incompatible with "packaging.version.Version" (reportArgumentType)

Mypy:

error: Argument 1 to "foo" has incompatible type "pkg_resources._vendor.packaging.version.Version"; expected "packaging.version.Version"  [arg-type]

The re-exported yield_lines method isn't typed either iirc. (Although that can be fixed upstream with a stub or TYPE_CHECKING block because of @functools.singledispatch)

So this is only related to the dynamic extern issue in that it would incidentally resolve part of it. But it's a different issue at its core that would still be present with statically vendored packages.

[Avasam]

(for the sake of this PR I'll split it off, but there's almost nothing left to do to mark pkg_resources as py.typed, so that's definitely something I'd like to handle as soon as all PRs linked in this PR's description are merged)

[abravalheri]

Thank you!