Allow `types.GeneratorType` to be used instead of `collections.abc.Generator` as the return type of generator functions

Feature

Currently, mypy only allows collections.abc.Generator and its supertypes as the return type of generator functions, and as a result, it cannot infer the correct return type for awaitable objects that define the __await__() method via types.GeneratorType:

import asyncio

from collections.abc import Generator
from types import GeneratorType
from typing import Never, reveal_type


class A:
    def __await__(self) -> GeneratorType[Never, Never, bool]:
        return True
        yield  # generator definition


class B:
    def __await__(self) -> Generator[Never, Never, bool]:
        return True
        yield  # generator definition


async def main() -> None:
    reveal_type(await A())  # revealed type is "Any"
    reveal_type(await B())  # revealed type is "builtins.bool"

    reveal_type(A().__await__().gi_running)  # revealed type is "builtins.bool"
    reveal_type(B().__await__().gi_running)  # has no attribute "gi_running"


if __name__ == "__main__":
    asyncio.run(main())

example.py:9: error: The return type of a generator function should be "Generator" or one of its supertypes  [misc]
example.py:21: note: Revealed type is "Any"
example.py:22: note: Revealed type is "builtins.bool"
example.py:24: note: Revealed type is "builtins.bool"
example.py:25: error: "Generator[Never, Never, bool]" has no attribute "gi_running"  [attr-defined]
example.py:25: note: Revealed type is "Any"
Found 2 errors in 1 file (checked 1 source file)

I propose allowing the use of types.GeneratorType with subsequent inference of the correct return type.

Pitch

First, pyright already has the expected behavior:

/home/user/workspace/example.py
  /home/user/workspace/example.py:21:17 - information: Type of "await A()" is "bool"
  /home/user/workspace/example.py:22:17 - information: Type of "await B()" is "bool"
  /home/user/workspace/example.py:24:17 - information: Type of "A().__await__().gi_running" is "bool"
  /home/user/workspace/example.py:25:17 - information: Type of "B().__await__().gi_running" is "Unknown"
  /home/user/workspace/example.py:25:33 - error: Cannot access attribute "gi_running" for class "Generator[Never, Never, bool]"
    Attribute "gi_running" is unknown (reportAttributeAccessIssue)
1 error, 0 warnings, 4 informations

Second, this relates to the eternally controversial python/typing#1480. In python/typeshed#10816, false properties related to native objects were removed in favor of the types module. You could say that we can use isinstance/inspect.isgenerator to narrow collections.abc.Generator down to types.GeneratorType, but... it is not that simple:

    x = B().__await__()
    reveal_type(x)  # typing.Generator[Never, Never, builtins.bool]

    if isgenerator(x):
        reveal_type(x)  # types.GeneratorType[Any, Any, Any]

    if isinstance(x, GeneratorType):
        reveal_type(x)  # types.GeneratorType[Any, Any, Any]

I think that allowing the use of types.GeneratorType would make it easier to inspect native generator objects. Especially since collections.abc.Generator can also be used to define custom generator types.

Related: #18635.

Start with the example.py case in the issue and compare mypy's current diagnostics with the expected pyright behavior. The work is done when types.GeneratorType is accepted as a generator-function return type and the shown await and generator attribute types are inferred as expected.