Mypy: Recognize dynamic creation of named tuples.

[ilevkivskyi]

This appeared in https://github.com/python/typing/issues/427

We can support something like this:

base_fields = [('x', int), ('y', int)]
label = [('label', str)]
LabeledPoint3D = NamedTuple('LabeledPoint3D', base_fields + [('z', int)] + label)

[ilevkivskyi]

We can also do similar things for TypedDict and Enum functional APIs.

[elliott-beach]

Adding this would also fix #4287.

[Dentosal]

Currently dynamically created enums also show errors, because mypy cannot resolve variants.

[msullivan]

I don't think we'll do this