Regression on master branch: false-positive error for enum mixin data types #12258

Using the current master branch, mypy reports a false positive error for the following snippet of code:

from enum import Enum

class MyEnum(Enum): ...

class BytesEnum(bytes, MyEnum): ...  # error: Only a single data type mixin is allowed for Enum subtypes, found extra "enum_test.MyEnum"

The error is not reported using mypy 0.931.

Expected behaviour

There should be no error reported; this works fine at runtime (and this inheritance structure is in fact used within enum.py in the CPython stdlib).

Related:

• Basic support for Python 3.11's typeshed #12220
• Fix several mypy errors when run on the 3.11 stdlib typeshed#7389

cc. @sobolevn