Reject types.NoneType and suggest using None instead

[JukkaL]

types.NoneType is the same as type(None) at runtime, and mypy doesn't recognize this. However, we don't want multiple was of representing None in type annotations, so we should generate an error if types.NoneType is used in a type annotation.

Hints:

• Fix this in mypy.typeanal
• Currently types.NoneType is treated as an mypy.types.Instance type but None is mapped to mypy.types.NoneType
• If we'd create an Instance with fullname equal to types.NoneType, generate an error

Example illustrating the problem:

from types import NoneType

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

f(None)   # Incompatible type None, expected NoneType

[AlexWaygood]

This is a similar issue to #11288 :)

[JukkaL]

Closed as dupe of #11288.