Added support for Concatenate with Ellipsis

[Daraan]

Fixes #110.

This PR adds support for Ellipsis in Concatenate for the Python 3.8-3.10 backports.

Does not fix:

• 48 : Using Ellipsis as argument for Callable with a generic..

  • <= 3.9: because of typing.Callable limitations
  • For 3.10 could go back to typing_extentions._ConcatenateGenericAlias but also tricky.

Changes:

• Changed _concatenate_getitem to reflect the Python3.11 typing module that allows the Ellipsis parameter.
• For Python 3.10 uses typing._ConcatenateGenericAlias and _concatenate_getitem
• For <=Python 3.9 the typing_extensions._ConcatenateGenericAlias and _concatenate_getitem like before
• For <3.9.2 uses a Dummy class that is temporarily swapped with an Ellipsis input
• Added tests

[cpython-cla-bot[bot]]

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[Daraan]

I've made a rework (moved the parts to _create_concatenate_alias) that is now simpler and also works for Python 3.8 and fully solves #110 on the current version range.

[AlexWaygood]

Looks like you might have closed this accidentally. Probably this is something to do with the fact that it was a PR from your fork's main branch. I think it's unfortunately impossible to repair PRs when they get to this state — do you want to recreate the PR using a different branch on your fork? :-)

[Daraan]

Thanks for your kind reply. Yep I've just butchered this 🙈, I am sorry. It's as you assume related to the fork from main ...

[AlexWaygood]

We've all been there, pretty sure I've done it at least once :-)