Marking a private Enum member as member() does not work #18563

Consider this example:

from enum import Enum, member

class Pet(Enum):
    __CAT = member(1)

reveal_type(Pet.__CAT)  # N: Revealed type is "enum.member[Literal[1]?]"

Link: https://mypy-play.net/?mypy=latest&python=3.12&gist=63b056215c22ba7ff09e2559dd2ae08a

However, in runtimes it fails with AttributeError, because __CAT is mangled to _Pet__CAT as regular private names are.

We need to fix this corner-case.

Refs #18491
Refs #18557