Problem: When an exception occurs in a WITH block, __exit__ was not called,
preventing proper cleanup of context managers.
Solution:
1. Wrap WITH block body in try-except structure
2. On normal exit: call __exit__(None, None, None)
3. On exception: call __exit__ with exception info before re-raising
Changes:
- compiler.c: Wrap WITH body in try-except, ensure __exit__ called in both paths
- ceval.c: Update OP_WITH_EXIT to accept three arguments (exc_type, exc_val, exc_tb)
- tests/520_context.py: Add test to verify __exit__ called on exceptions
