Treating NoneType as None and warning against using NoneType.

[strny0]

Description

Fixes #11288

Warns people to not use NoneType as a type annotation, as requested in #11288.

Example

Before

from types import NoneType

def f(x: NoneType) -> None:
    pass

f(None) # E: Argument 1 to "f" has incompatible type "None"; expected "NoneType"

After

from types import NoneType

def f(x: NoneType) -> None: # E: NoneType should not be used as a type, please use None instead 
    pass

f(None)

Had to edit test-data/unit/lib-stub/types.pyi to allow NoneType type annotation.

[hauntsaninja]

Thanks, this lgtm

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉