[FEAT] Track and support Python 3.15

Python 3.15 is scheduled for release in October. To ensure seamless compatibility upon release, we should track and address any compatibility issues, compiler warnings, or build errors.

Tasks

• #1274
• Verify that all C extensions compile cleanly without new deprecation warnings.
• Address any module deprecations or C API changes introduced in Python 3.15 (following the Python 3.15 What's New document).
• Ensure that dependency constraints allow 3.15.