Fix intransitive subtyping issue with SupportsGetItem

[hauntsaninja]

See python/mypy#13713 for details

[AlexWaygood]

... why is typing.Container even generic in the first place??

[hauntsaninja]

Maybe it once had a strict annotation for __contains__? Maybe it just felt aesthetically wrong to have a container that wasn't generic. Didn't have time to spelunk. In an ideal world it would be removed, but I assumed it'd be too breaking to touch.

[AlexWaygood]

I guess it is pretty nice to say something's a Container[str] if it contains strings, even if it is meaningless to the type checker

[AlexWaygood]

It might be nice to add a comment to typing.pyi noting the oddity, though

[hauntsaninja]

I did a spelunk, and it's always been this way. From before typeshed:
337abed#diff-009a41a8f985ef690b94fb876cfcd4d00fff765eb3f41468a1d816c555bc338bR91
And from the earliest days of mypy:
python/mypy@05f70ca#diff-e114935a16df41cf681c93cd8be4e3b681194568d45c54f4df7985e149395dbcR265

[AlexWaygood]

I did a spelunk, and it's always been this way. From before typeshed: 337abed#diff-009a41a8f985ef690b94fb876cfcd4d00fff765eb3f41468a1d816c555bc338bR91 And from the earliest days of mypy: python/mypy@05f70ca#diff-e114935a16df41cf681c93cd8be4e3b681194568d45c54f4df7985e149395dbcR265

We did simultaneous spelunking, I arrived at the same conclusion :-)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[AlexWaygood]

Looks good!