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.

Beginne mit dem Reproducer im Issue und verfolge, wie mypy die funktionale API von Enum für Enum-Unterklassen verarbeitet. Prüfe, wie der abgeleitete Typ von Foo('Baz', ['X', 'Y']) und Annotationen, die Baz verwenden, verarbeitet werden. Als abgeschlossen gilt die Aufgabe, wenn sowohl der Factory-Aufruf der Unterklasse als auch Mitglieder vom Typ Baz ohne die gemeldeten Fehler typgeprüft werden.