Backport allowing passing an ellipsis as the last argument of Concatenate to 3.10

[FasterSpeeding]

python/cpython#30969 allowed and documented this behaviour for 3.11.

This seemingly can't be supported for <=3.9 according to #48 but changes made by python/cpython#30959 to the stdlib typing module lead to this behaviour erroring on later 3.10 releases when using typing_extensions.Concatenate.

[JelleZijlstra]

This is now documented as a limitation at https://typing-extensions.readthedocs.io/en/latest/#Concatenate. If there is a reasonable way to fix it, I'd accept a PR.