Fix #20821: Check __qualname__ assignment type in class body

[BHUVANSH855]

Fixes #20821

This PR adds validation for assignments to __qualname__ inside class bodies.

Previously, mypy allowed assigning non-string values to __qualname__, even though this raises a TypeError at runtime in Python. This PR aligns mypy’s behavior with CPython by enforcing that __qualname__ must be a string.

Changes

• Added a semantic analysis check for assignments to __qualname__ in class scope
• Emit an error if the assigned value is not a string
• Added a test case to cover this behavior

Example

class X:
    __qualname__ = 5  # Now correctly produces an error

Notes

• The check is implemented in semantic analysis (semanal.py)
• The change is minimal and only affects class-level assignments to __qualname__

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

dd-trace-py (https://github.com/DataDog/dd-trace-py)
+ ddtrace/debugging/_function/discovery.py:46: error: "__qualname__" must be str  [misc]