Generic MRO

Generic information are lost with classes __mro__. But it could be useful to have it, especially for #776 implementation.
typing module could integrate a generic_mro function in order to retrieve the class MRO annotated with related generic information; this function could also accept a generic alias in argument.

from typing import Generic, TypeVar, generic_mro

T = TypeVar("T")
class A(Generic[T]): ...

U = TypeVar("U")
class B(A[U]): ...

class C(B[int]):
    pass

assert generic_mro(C) == (C, B[int], A[int], Generic, object)
assert generic_mro(B) == (B, A[U], Generic, object)
assert generic_mro(B[int]) == (B[int], A[int], Generic, object)

首先阅读提议的 generic_mro API 和相关 issue #776；该 issue 没有指出任何实现文件或测试。完成的标准是提供提议的函数，并匹配针对 C、B 和 B[int] 所记录的三个 MRO 结果。