Add missing _add_value_alias_ method for Python 3.13 enum

[tjdcs]

Summary

• Add missing _add_value_alias_ method to Enum class for Python 3.13
• Add comprehensive test cases for the MultiValueEnum pattern

Fixes python#14408 by resolving the type checker error when using _add_value_alias_ method in Python 3.13 enum classes.

Changes Made

• Added _add_value_alias_(self, value: Any) -> None method to Enum class in stdlib/enum.pyi
• Added proper Python 3.13 version guard (if sys.version_info >= (3, 13):)
• Added comprehensive test cases in stdlib/@tests/test_cases/check_enum.py including:
  • MultiValueEnum pattern from Python documentation
  • Type inference tests for primary and alias values
  • Literal type verification

Test Plan

• Verify Pyright recognizes the method without errors (0 errors, 0 warnings)
• Verify mypy recognizes the method without errors (Success: no issues found)
• Test runtime functionality with Python 3.13 (✅ Works correctly)
• Ensure no regressions in existing enum functionality (✅ All tests pass)
• Test cases pass with both type checkers (✅ Pyright and mypy both succeed)

Example Usage

from enum import Enum

class MultiValueEnum(Enum):
    def __new__(cls, value, *values):
        self = object.__new__(cls)
        self._value_ = value
        for v in values:
            self._add_value_alias_(v)  # No longer causes type error
        return self

class DType(MultiValueEnum):
    float32 = 'f', 8
    double64 = 'd', 9

# Both work correctly
print(DType('f'))  # <DType.float32: 'f'>
print(DType(8))    # <DType.float32: 'f'>

🤖 Generated with Claude Code

[tjdcs]

Closing this PR as it was created against the wrong repository. The correct PR is now at python#14411