Infra: makefile/RTD: Use uv if installed

[hugovk]

Like python/devguide#1320.

Using uv to install dependencies is quicker than pip.

On Read the Docs, it adds 1 second to install uv, and then cuts the docs build time from 134s -> 115s, overall reduces the time from 146s -> 129s.

pip: https://beta.readthedocs.org/projects/hugovk-peps/builds/24491058/
uv: https://beta.readthedocs.org/projects/hugovk-peps/builds/24491059/

Locally, running make clean; time make venv with a warm cache goes from 8.6s -> 0.5s:

• pip: make venv 4.49s user 0.94s system 63% cpu 8.586 total
• uv: make venv 0.04s user 0.12s system 34% cpu 0.461 total

make html is the same for both.

Overall, make clean html goes from 1m26s -> 1m09s.

---

📚 Documentation preview 📚: https://pep-previews--3791.org.readthedocs.build/