Refactor and fix pylance

- Refactor all files to fix pylance issues - Modularized Ass.__init__ - Update pyproject to use the latest VideoTimestamps version - Fix ass.input_timestamps that could possibly be None before in Ass class - Deprecate Python 3.9 support from the library (we are already at Python 3.13, and this way we could follow better PEP 604/585)