Generalise attribute __func__ of class MethodType from FunctionType to Callable

[tyralla]

Fixes #11200

[github-actions]

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

[srittau]

The comment of _StaticFunctionType hints at an old (2017) mypy hack, although it seems the implementation also uses a member_descriptor class. Did you try just changing the return type of __get__ to a Callable?

[tyralla]

No, I did not because _StaticFunctionType seems unnecessary when returning Callable instead of FunctionType. However, if you think my change could cause unexpected trouble, we could also follow your suggestion, which leads to the same result in the following example (MethodType2 vs. MethodType4):

from types import FunctionType
from typing import Any, Callable

class MethodType1:
    @property
    def __func__(self) -> FunctionType: ...

class MethodType2:
    @property
    def __func__(self) -> Callable[[...], Any]: ...

class Descriptor3:
    def __get__(self, obj: object, type: type | None) -> FunctionType: ...

class MethodType3:
    @property
    def __func__(self) -> Descriptor3: ...

class Descriptor4:
    def __get__(self, obj: object, type: type | None) -> Callable[[...], Any]: ...

class MethodType4:
    @property
    def __func__(self) -> Descriptor4: ...

reveal_type(MethodType1().__func__)  # note: Revealed type is "types.MethodType"
reveal_type(MethodType2().__func__)  # note: Revealed type is "def (Any) -> Any"
reveal_type(MethodType3().__func__)  # note: Revealed type is "types.FunctionType"
reveal_type(MethodType4().__func__)  # note: Revealed type is "def (Any) -> Any"