Support custom subclasses of `EnumType`

[erictraut]

The EnumType metaclass can be used to create custom Enum classes. Pyright doesn't currently recognize these as enums.

from enum import EnumType
from typing import Literal, assert_type

class CustomEnumType(EnumType):
    pass

class CustomEnum(metaclass=CustomEnumType):
    pass

class Color(CustomEnum):
    RED = 1
    GREEN = 2
    BLUE = 3

assert_type(Color.BLUE, Literal[Color.BLUE])

[erictraut]

This is addressed in pyright 1.1.348, which was just published. It will also be included in a future release of pylance.