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

Start with the get_type_hints API and the generic-class examples in the issue, then review how generic aliases and TypeVar relationships are currently represented. Done means defining and implementing the proposed substitution behavior, including the default-disabled parameter and alias inputs, with tests covering B, B[T], and B[int].