typing: add _ProtocolMeta

[hauntsaninja]

It looks like a couple people are using this in the wild.

Resolves #6393

[sobolevn]

+1, I use it quite a lot.
The first usecase I can remember and find: https://github.com/antonagestam/phantom-types/blob/main/src/phantom/sized.py#L27-L34

I guess we also need to add it to typing_extensions then.

[hauntsaninja]

Okay, some notes:
_ProtocolMeta has been in typing.py since the beginning. However, in Python 3.7, it was class _ProtocolMeta(type): .... Since the use case for this internal thing is so you can have your own metaclasses for Protocols, it thus doesn't make sense to have it at all before Python 3.8.

Btw, for whatever it's worth, it looks like that phantom types use case isn't really affected by the issue linked, since the metaclasses it's using don't have any attributes that contribute to the protocol. If it were to add attributes, it might still run into issues, because in general mypy doesn't understand try: ... except ImportError: ...

[github-actions]

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

[Akuli]

No red flags in code, and the example code from related issue no longer errors.