Make typing.Coroutine a protocol

[AlexWaygood]

This was in the original diff for #10816 but got removed before the PR was merged -- it looks like the removal might have been an accidental change as part of a merge commit. At runtime isinstance() checks against this class use duck-typing, and we already have typing.Generator as a protocol:

>>> class MyCoro:
...     def __await__(self): ...
...     def throw(self): ...
...     def send(self): ...
...     def close(self): ...
...
>>> from collections.abc import Coroutine
>>> isinstance(MyCoro(), Coroutine)
True

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/utilities/asyncutils/__init__.py:361: error: Incompatible return value type (got "_Wrapped[P, Coroutine[Any, Any, Any], [VarArg(Any), DefaultNamedArg(bool | None, '_sync'), KwArg(Any)], Coroutine[Any, Any, Any]]", expected "Callable[P, R? | Coroutine[Any, Any, R?]]")  [return-value]
+ src/prefect/utilities/asyncutils/__init__.py:361: error: Incompatible return value type (got "_Wrapped[P, Coroutine[Any, Any, R?], [VarArg(Any), DefaultNamedArg(bool | None, '_sync'), KwArg(Any)], Coroutine[Any, Any, R?]]", expected "Callable[P, R? | Coroutine[Any, Any, R?]]")  [return-value]
- src/prefect/utilities/asyncutils/__init__.py:361: note: "_Wrapped[P, Coroutine[Any, Any, Any], [VarArg(Any), DefaultNamedArg(bool | None, '_sync'), KwArg(Any)], Coroutine[Any, Any, Any]].__call__" has type "def __call__(*args: Any, _sync: bool | None = ..., **kwargs: Any) -> Coroutine[Any, Any, Any]"
+ src/prefect/utilities/asyncutils/__init__.py:361: note: "_Wrapped[P, Coroutine[Any, Any, R?], [VarArg(Any), DefaultNamedArg(bool | None, '_sync'), KwArg(Any)], Coroutine[Any, Any, R?]].__call__" has type "def __call__(*args: Any, _sync: bool | None = ..., **kwargs: Any) -> Coroutine[Any, Any, R?]"

[AlexWaygood]

these aren't required for an isinstance() check to pass at runtime, and I don't think __name__ or __qualname__ attributes are really relevant to the abstract idea of what it means for a nominal class (like types.CoroutineType) to be coroutine-like. So I don't think it's appropriate for them to be listed here.

types.CoroutineType, the concrete, nominal type returned by async def functions at runtime, should have these attributes, of course, and still does on this branch:

typeshed/stdlib/types.pyi

Lines 428 to 431 in 57035b1

	@final
	class CoroutineType(Coroutine[_YieldT_co, _SendT_nd_contra, _ReturnT_nd_co]):
	__name__: str
	__qualname__: str