Test upcoming changes on packaging

[abravalheri]

Packaging is preparing for a new release (see pypa/packaging#1063), so I want to run the CI to check if everything is going to be OK...

The assumptions are the following:

• Setuptools uses sys.path.extend to add the _vendor folder to the path...
• So this should mean that it goes after the regular path
• Any other package already existing in the regular path will be loaded from there, not from _vendor.
• By adding packaging>=26.0rc3 to the tests dependency, we ensure that packaging==26.0rc3 is loaded from the the regular site-packages (not from the _vendor folder)
• This should be enough to test against the newer version of packaging instead of the vendored one...

Summary of changes

Closes

Pull Request Checklist

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

[abravalheri]

PyPy has been very unstable lately...

Anyway, it seems to me that the code is compatible with the upcoming release of packaging.