Add audioop-lts for Python 3.13

[Mirochill]

Summary

• add audioop-lts for Python 3.13+ so pydub can import the removed stdlib audioop module
• keep the dependency conditional so Python 3.11/3.12 environments are unchanged
• update both pyproject.toml and requirements.txt

Fixes #206

Validation

• git diff --check
• Not run locally: tests/install were not run