Allow `NotRequired[]` to be passed as a `TypeVar`

I just found myself wanting to do the following:

from typing import TypedDict, NotRequired, Generic, TypeVar, Union, Never

_T = TypeVar('_T')

Foo = Union['FooSub1[_T]', 'FooSub2[_T]', 'FooSub3[_T]']

class BaseFoo(TypedDict, Generic[_T]):
    bar: _T

class FooSub1(BaseFoo[_T]):
    sub1: str

class FooSub2(BaseFoo[_T]):
    sub2: str

class FooSub3(BaseFoo[_T]):
    sub3: str

def foo(foo_with_bar: Foo[int]):
    reveal_type(foo_with_bar) # Type of "foo_with_bar" is "FooSub1[int] | FooSub2[int] | FooSub3[int]"

def bar(foo_without_bar: Foo[NotRequired[Never]]): # error: "NotRequired" is not allowed in this context
    pass

Mypy and Pyright both disallow NotRequired to be passed as a TypeVar. However the only way to avoid doing that would require me to duplicate all subclass definitions, which I am not willing to do because I have more than a dozen and so much duplication would make the code less maintainable.

Start with the reproducer in the issue and compare the current NotRequired and TypeVar rules in mypy and Pyright, the two checkers named in the report. No files or tests are identified; done would require an agreed typing-rule design and corresponding checker behavior without duplicating the TypedDict subclasses.