Fix __init__ type of typing.NamedTuple #6079

While working on python/mypy#11047 I've noticed that there's an issue with typing.NamedTuple

Right now __init__ of typing.NamedTuple has type: def __init__(self, typename: str, fields: Iterable[Tuple[str, Any]] = ..., **kwargs: Any) -> None: ... which is not correct.

Why? Let's try to add both fields and kwargs:

>>> from typing import NamedTuple
>>> NamedTuple('a', (), f=int)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/sobolev/.pyenv/versions/3.9.1/lib/python3.9/typing.py", line 1864, in NamedTuple
    raise TypeError("Either list of fields or keywords"
TypeError: Either list of fields or keywords can be provided to NamedTuple, not both

Source: https://github.com/python/cpython/blob/f56268a2cd38b3fe2be1e4361d3d8b581e73559b/Lib/typing.py#L2290-L2294

So, it should be an overload instead:

    @overload
    def __init__(self, typename: str, fields: Iterable[Tuple[str, Any]] = ...) -> None: ...
    @overload
    def __init__(self, typename: str, **kwargs: Any) -> None: ...

Notice, that fields=None is still allowed:

>>> NamedTuple('a', fields=None, f=int)

But, I don't think that we should have def __init__(self, typename: str, fields: None = ..., **kwargs: Any) -> None: ... instead.

One more thing: is it Tuple[str, Any] or Tuple[str, type]? It is supposed to be types. But, it is not enforced anywhere (except mypy) as far as I know.

I will send a PR with the fix.