Fix for TypeVar bound to a union and methods returning Self

[mariafnafees]

This pull request resolves #18170 by addressing issues related to TypeVar with Union bounds and methods returning Self. It includes changes to improve type inference and ensuring compatibility with the current logic.

Logic Fixes

• Enhanced type inference for TypeVar bound to Union types, ensuring proper narrowing for methods returning Self.
• Updated checkmember.py and typeanal.py to address issues involving get_proper_type() and UnionType.

Enhancements

• Improved analyze_type_alias() in typeanal.py to correctly handle TypeVar bound to Union, preserving type relationships in Self-returning methods.
• Refined analyze_member_access() in checkmember.py to ensure accurate handling of Self with TypeVar and Union.

Test Cases

• Added testTypeVariableSelfUnion and testTypeVarUnionWithSelf to validate the addition of the new features.
• Enhanced the test suite to include edge cases for TypeVar and Union.

CI Compliance

• All changes verified to pass runtests.py and other tests in the CI pipeline.

Improvements to Mypy

• This pull request improves Mypy's ability to handle TypeVar bound to Union types, particularly when interacting with methods returning Self.

The changes ensure:

• Proper type inference for complex relationships between TypeVar and Union.
• Accurate narrowing of types based on runtime behavior in methods like clone()

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅