gh-121291: Respect mixin bitwise-operator overrides on Flag subclasses (GH-155862)

* gh-121291: respect mixin-defined bitwise operators on Flag subclasses

EnumMeta.__new__ unconditionally installed Flag's __or__/__and__/__xor__/
__ror__/__rand__/__rxor__/__invert__ onto every Flag subclass, silently
discarding a mixin base's own override of these dunders -- even though
the class's MRO should have resolved to the mixin's method; this fixes that.