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.

从 issue 中的复现代码开始，比较报告中提到的两个检查器 mypy 和 Pyright 当前对 NotRequired 和 TypeVar 的规则。没有确定任何文件或测试；完成这项工作需要达成一致的类型规则设计以及相应的检查器行为，同时不重复 TypedDict 子类。