Release and declare support for Python 3.13

In particular, Python 3.13 is not presently listed in the classifiers in [`pyproject.toml`](https://github.com/MagicStack/asyncpg/blob/master/pyproject.toml#L13-L29).

Lack of documented support is preventing updates to Python 3.13 on projects for which `asyncpg` is a production (non-development) dependency.