Prefer `__new__` over `__init__` for determining callable type of a class

[msullivan]

#5642 made __new__ chosen in more situations but __init__ is still chosen when
one class defines both. The difficulty is that typeshed has a bunch of dodgy __new__
annotations that need to be cleaned up first.

[hauntsaninja]

Here's an example (simplified from a post in the gitter today) where we get a terrible error message because IntEnum subclasses int.

~/delete λ cat test34.py
from enum import IntEnum

IntEnum("Fruit", "Apple Pear")
~/delete λ mypy test34.py
test34.py:3: error: No overload variant of "IntEnum" matches argument types "str", "str"
test34.py:3: note: Possible overload variant:
test34.py:3: note:     def IntEnum(self, x: Union[str, bytes, bytearray], base: int) -> IntEnum
test34.py:3: note:     <1 more non-matching overload not shown>
Found 1 error in 1 file (checked 1 source file)

Curious if you remember what the problematic typeshed __new__s are?