Fix enum private names (__x) incorrectly marked as final

[Fridayai700]

Summary

Since Python 3.11, private names (names starting with __ but not ending with __, like __config) are not enum members and should not be treated as final. Currently, mypy incorrectly marks them as final, causing false positives.

Before:

from enum import Enum

class MyEnum(Enum):
    __config = "some_value"
    A = 1
    B = 2

MyEnum._MyEnum__config = "other_value"  # error: Cannot assign to final attribute "__config"

After: No error — private names are not enum members and can be reassigned.

Root cause

In semanal.py, the check for non-final enum names used is_dunder() which only matches __x__ (dunder names). Private names like __config were missed.

Meanwhile, TypeInfo.enum_members in nodes.py already correctly used name.startswith("__") to exclude both dunders AND private names from the member list. This created an inconsistency: __config was not listed as a member but was still treated as final.

Fix

Changed the finality check to use name.startswith("__"), matching the existing enum_members logic. This ensures consistency between member detection and finality marking.

Test plan

• Added testEnumPrivateNameNotFinal test case
• All 180 existing enum tests pass
• python -m pytest mypy/test/testcheck.py -k Enum -o "addopts=" — all pass

Fixes #20789