TypeVar substitution in get_type_hints

When it comes to generic classes, type annotations retrieved with get_type_hints are not really easy to use, as in the following example:

from typing import Generic, TypeVar, get_type_hints

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

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

assert get_type_hints(B) == {"a": T, "b": U}

It could be useful to add a TypeVar substitution to get_type_hints, for example with an additional substitute_type_vars parameter (with a False default)
By the way, get_type_hints could also allow to pass generic alias in order to substitute directly TypeVars with their related argument.

It would give for the example above:

assert get_type_hints(B, substitute_type_vars=True) == {"a": U, "b": U}  # typevars are consistent
assert get_type_hints(B[T], substitute_type_vars=True) == {"a": T, "b": T}
assert get_type_hints(B[int], substitute_type_vars=True) == {"a": int, "b": int}  # ready to use

从 get_type_hints API 和 issue 中的 generic-class 示例开始，然后检查当前如何表示 generic aliases 和 TypeVar 关系。完成标准是定义并实现提议的替换行为，包括默认禁用的参数和 alias 输入，并添加涵盖 B、B[T] 和 B[int] 的测试。