Test Python 3.13 and use 3.12 to build sdist and docs

[hugovk]

I added 3.13 to tox and the CI, then ran tox -e py313 and for each package that needed upgrading for Python 3.13, ran something like:

pip-compile --generate-hashes --output-file=requirements.txt requirements.in --upgrade-package aiohttp

I also used pip-tools on Python 3.12 because we have 3.12 on the downloads and docs servers.

Then also bumped to use Python 3.12 to build the sdist and docs on the CI.

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I didn't add 3.14 for testing yet, because we want to use --only-binary and there's some things which don't have 3.14 binaries yet (coverage was the first).