Support `types.new_class`

Feature

Currently, types.new_class is not supported.

Source: https://github.com/python/cpython/blob/9fa930dd4859441d36e39ec6db1a6ebdc046d5b1/Lib/types.py#L69
Typeshed: https://github.com/python/typeshed/blob/ca983cd319cb58b0a8fa3a086fad5e75332e0450/stdlib/types.pyi#L344-L359

Pitch

It creates inconsistency with CPython for mypy:

Example:

from types import new_class

X = new_class('X', (int,))

def sum_two(a: X, b: X) -> X:
    return a + b

print(sum_two(X(2), X(3)))

There are two ways we can do this:

1. Via plugin get_dynamic_class_hook(), which looks like it was created for this case! (my preference)
2. Via custom semanal_new_class helper

Any tips / preferences from maintainers here?

Start by comparing the referenced CPython Lib/types.py and typeshed types.pyi definitions for new_class. Then investigate mypy's get_dynamic_class_hook() entry point and the proposed semanal_new_class helper. Done means the provided example type-checks consistently with CPython behavior, with the implementation approach resolved.