Remove pkg_resources

[jaraco]

• Remove pkg_resources
• Remove assertion that 'import pkg_resources' works; it's no longer an invariant.
• Mark pbr integration test as xfail.
• Mark namespace tests as xfail. Support for pkg_resources-style namespaces is dropped.
• Add news fragment.

Summary of changes

Closes #3085

Pull Request Checklist

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

[jaraco]

To be sure, this change is a big deal. It's been decades in the making to provide replacements for this functionality. There are known open issues and efforts to remove the dependence on pkg_resources and probably other use-cases that haven't migrated. My hope is that these cases can pin to Setuptools<81.

I don't have any plans to rush this out or force it on the community, but I'd like to start thinking about what it would take to get committed.

[abravalheri]

Thank you very much Jason for the effort in untangling the 2 code bases.
I agree that it is time to start thinking about the removal of pkg_resources.

I propose we do it in 2 stages:

1. Explicitly mention a "due date" to the warning message
2. Removal once the "due date" arrives.

---

I have 2 doubts:

• Does the namespace_packages (deprecated) keyword for setup.py works without pkg_resources.declare_namespace?

  If I understood correctly previously these worked in tandem, but I don't know what is the situation now after the latest changes in https://github.com/pypa/setuptools/blob/v80.8.0/setuptools/namespaces.py#L47-L63.

• Is the idea of https://github.com/pypa/pkg_resources to also publish something to PyPI? Even if the package is born deprecated it may be a "last-resort" alternative for users that rely on packages that have not received updates for a long time, or that are still working on the migration...

  I know at least that folks from zope/plone have been slowly trying to replace pkg_resources.declare_namespace for a while, but are not there quite yet (refs: Should we add a DeprecationWarning to pkg_resources.declare_namespace? #3434 (comment), Switch to PEP 420 (Implicit namespace packages) zopefoundation/meta#194), so maybe that would be useful if they take can install a package to extend the time they can work on the migration. We could also consider adding pkg_resources in setuptools.build_meta:__legacy__.get_requires_for_build_sdist if we want to be very conservative about the transition.

  Update: I can see in Setuptools should not rely on pkg_resources #3085 (comment) that this is not straight forward.