bpo-43224: Implement pickling of TypeVarTuples

[mrahtz]

Guess who learned a whole bunch about how pickling works :)

Notes:

• I was a bit horrified to realise that the class I'd renamed to _BoundVarianceMixin also contains a key ingredient for pickling of things that use it! I've refactored its __reduce__ into a separate mixin for clarity.
• I'm not entirely sure whether these tests make sense - is assertEqual a reasonable way of testing this functionality? Am I correct in thinking we do need to test both assertEqual and assertIs for TypeVarTuples and unpacked TypeVarTuples?

This PR doesn't include implementation of pickling support for unpacked native tuple; I'll do that in a future PR. (And I guess we'll also need to add tests for copy?)

https://bugs.python.org/issue43224

[Fidget-Spinner]

correct in thinking we do need to test both assertEqual and assertIs for TypeVarTuples and unpacked TypeVarTuples?

IIRC, assertIs for pickle/unpickling isn't guaranteed, so we don't need to test for that.

Other than the tests, the code in typing.py LGTM.

[JelleZijlstra]

Thanks, I think these tests are a bit repetitive.

[Fidget-Spinner]

LGTM. Thanks!

[Fidget-Spinner]

Why was the PR closed @mrahtz ?

Edit: Nevermind, I just read the bug report.

[mrahtz]

Lol, my bad - I was cleaning up some old branches, and forgotten this hadn't yet been merged 😅

[mrahtz]

Thanks, Jelle!