Removed `typing_extensions` from vendored dependencies

[Avasam]

Summary of changes

Removed typing_extensions from vendored dependencies.

All vendored dependencies put typing_extensions behind a TYPE_CHECKING block, in a stub file, or behind a Python 3.8+ check. Hence it is no longer necessary to vendor typing_extensions. (and if a vendored library re-introduce a problem importing it at runtime, we can either raise an issue upstream and wait for the next version, or re-introduce a patch to make it conditional rather than use a vendor version).

Closes #4324

Pull Request Checklist

• [x] Changes have tests (expecting existing tests to cover this change)
• [x] News fragment added in newsfragments/. (See [documentation][PR docs] for details)

[PR docs]: https://setuptools.pypa.io/en/latest/development/developer-guide.html#making-a-pull-request

[abravalheri]

Thank you very much @Avasam. I am in favour of simplifying as much as we can the vendoring.

There is a risk it might reappear when we update the vendored dependencies, but should be good for now.