Should we use `_ProtocolMeta` in `typing.py`?

[sobolevn]

Right now _ProtocolMeta is never used:

typeshed/stdlib/typing.pyi

Line 282 in 739711e

class _ProtocolMeta(ABCMeta): ...

But, it is a base metaclass for Protocol type in typing.py: https://github.com/python/cpython/blame/723ebe76e787cfa6b08cc9587dd679f3234a1025/Lib/typing.py#L1965

It is also used for all Protocol types (we currently just use ABCMeta for them`):

typeshed/stdlib/typing.pyi

Lines 287 to 305 in 739711e

	@runtime_checkable
	class SupportsInt(Protocol, metaclass=ABCMeta):
	@abstractmethod
	def __int__(self) -> int: ...
	@runtime_checkable
	class SupportsFloat(Protocol, metaclass=ABCMeta):
	@abstractmethod
	def __float__(self) -> float: ...
	@runtime_checkable
	class SupportsComplex(Protocol, metaclass=ABCMeta):
	@abstractmethod
	def __complex__(self) -> complex: ...
	@runtime_checkable
	class SupportsBytes(Protocol, metaclass=ABCMeta):
	@abstractmethod
	def __bytes__(self) -> bytes: ...

CPython: https://github.com/python/cpython/blame/723ebe76e787cfa6b08cc9587dd679f3234a1025/Lib/typing.py#L2745

So, the question is:

1. Should we use for Protocol class? It is rather complex, because right now Protocol is _SpecialForm
2. Should we use it for Protocol types in typing.pyi?

[JelleZijlstra]

typing.pyi is central to type checking and typecheckers need to heavily special case for the contents of the stub. Therefore, I think we should only make changes here if they fix a concrete problem for users, not out of mere perfectionism.