Check consistency of __new__ and __init__ #5648

We permit the following program:

class A:
    def __init__(self, x: int) -> None:
        pass

class B(A):
    def __new__(cls) -> 'B':
        return object.__new__(cls)

B()

even though it crashes at runtime.

Per #5642 (review):
The rule CPython uses is this (for a class C with superclass B, where B is not object):

• If both C.__new__ and C.__init__ refer to their versions in object, the signature is def () -> C.
• If exactly one of C.__new__ and C.__object__ refers to its object version, the other determines the signature.
• If both override their corresponding object versions, they must match -- even if they don't come from the same class (say, if one comes from C and the other from B).