`MethodType` uses `FunctionLike` types inconsistently

[tyralla]

Since version 1.8, Mypy does not intersect types in isinstance checks if at least one is final. In my code base, this leads to unexpected unreachable warnings. I could narrow them down to:

from types import MethodType

m: MethodType
if isinstance(m.__func__, MethodType):  # error: Subclass of "FunctionType" and "MethodType" cannot exist: "FunctionType" is final  [unreachable] \
                                        # error: Subclass of "FunctionType" and "MethodType" cannot exist: "MethodType" is final  [unreachable]
    m = m.__func__  # error: Statement is unreachable  [unreachable]

As I implemented the Mypy change, I double-checked that I did not break something. But it seems the definition of MethodType in typeshed is inconsistent. Its __init__ method accepts Callable (#1499), but its __func__ attribute returns FunctionType (#1383). As MethodType can accept and return other methods at runtime, it seems right to define the return type as Callable, too.