Allow subclassing without supertyping

This issue comes from python/mypy#1237. I'll try to make a summary of the discussion here

Some one reported an issue with an artificial example (https://github.com/python/mypy/issues/1237#issue-136162181):

class Foo:
    def factory(self) -> str:
        return 'Hello'

class Bar(Foo):
    def factory(self) -> int:
        return 10

and then with the following stub files (https://github.com/python/mypy/issues/1237#issuecomment-188710925):

class QPixmap(QPaintDevice):
    def swap(self, other: 'QPixmap') -> None: ...

class QBitmap(QPixmap):
    def swap(self, other: 'QBitmap') -> None: ...

Which mypy currently reports as erroneous: Argument 1 of "swap" incompatible with supertype "QPixmap"

These were initially was argued to be a correct error because they violate Liskov Substitution Principle (the same case by a change of return type, the second is a covariant argument change). The problem in this scenario is that the actual classes are not meant to be substituted (the first example is actually a wrapper for a non-virtual C++ function so they're not substitutable, and the second aren't supposed to be mixed together, just reuse part of the code). (see https://github.com/python/mypy/issues/1237#issuecomment-189490903)

There was also a suggestion that allow covariant args explicitly (in the same way that Eiffel allows it and adds a runtime check) with a decorator

class QBitmap(QPixmap):
    @covariant_args
    def swap(self, other: 'QBitmap') -> None: ...

My proposal instead was add what some dialects of Eiffel call "non-conforming inheritance" ("conforms" is the Eiffel word for what PEP-483 calls "is-consistent-with"). It is essentially a mechanism to use subclassing just as a way to get the implementation from the superclass without creating a subtyping relation between them. See https://github.com/python/mypy/issues/1237#issuecomment-231199419 for a detailed explanation.

The proposal is to haven Implementation in typing so you can write:

class QBitmap(Implementation[QPixmap]):
    def swap(self, other: 'QBitmap') -> None: ...

which just defines a QBitmap class with all the mothods copied from QPixmap, but without making one a subtype of the other. In runtime we can just make Implementation[QPixmap] == QPixmap

Start by reading the linked mypy#1237 discussion and the referenced explanation of non-conforming inheritance. Compare the proposed Implementation[QPixmap] behavior with the existing subtype and method-override rules. The work is done only when the semantics, runtime behavior, and typing rules are agreed and covered by the project's implementation and tests.