Support functional API for Enum *Subclasses* too

Tested with version 0.641.

The functional API for enums is only supported on Enum itself, and not subclasses (that do not override the relevant methods). I hit this using the OrderedEnum suggestion from the standard library documentation: https://docs.python.org/3/library/enum.html#orderedenum.

Running on:

from enum import Enum

class Foo(Enum):
    ""

Bar: type = Enum('Bar', ['X', 'Y']) # line 6
Baz: type = Foo('Baz', ['X', 'Y']) # line 7

bar: Bar = Bar.X # line 9
baz: Baz = Baz.Y # line 10

gives

asdf.py:7: error: Incompatible types in assignment (expression has type "Foo", variable has type "type")
asdf.py:7: error: Too many arguments for "Foo"
asdf.py:10: error: Invalid type "asdf.Baz"
asdf.py:10: error: "type" has no attribute "Y"

Note that 6 is deemed fine, but 7 causes complaints. I'd hope for both lines to be fine.

The even larger issue is that annotations using Baz totally fail too: Line 9 is deemed fine but 10 fails. I wouldn't mind the obscure use of the Enum subclass failing if it weren't for the fact that every annotation with my nice enum type won't work.

Start with the reproducer in the issue and trace mypy's handling of the Enum functional API for Enum subclasses. Check how the inferred type of Foo('Baz', ['X', 'Y']) and annotations using Baz are processed. Done means both the subclass factory call and members typed as Baz type-check without the reported errors.