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

Bắt đầu bằng cách đọc cuộc thảo luận mypy#1237 được liên kết và phần giải thích được tham chiếu về kế thừa không tuân thủ. So sánh hành vi được đề xuất của Implementation[QPixmap] với các quy tắc hiện có về kiểu con và ghi đè phương thức. Công việc chỉ hoàn tất khi ngữ nghĩa, hành vi khi chạy và các quy tắc kiểu được thống nhất và được bao quát bởi phần triển khai cùng các bài kiểm thử của dự án.