makefile/RTD: Use uv if installed

[hugovk]

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 28s -> 13s, overall halves the time from 28s -> 14s.

• pip: https://beta.readthedocs.org/projects/hugovk-devguide/builds/24341331/
• uv: https://beta.readthedocs.org/projects/hugovk-devguide/builds/24342365/

Locally, running make clean; time make venv with a warm cache:

pip: make venv 4.05s user 0.92s system 91% cpu 5.451 total
uv: make venv 0.03s user 0.19s system 119% cpu 0.179 total

make html is the same for both.

Overall, make clean html goes from 7s -> 3s.

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📚 Documentation preview 📚: https://cpython-devguide--1320.org.readthedocs.build/

[hugovk]

And the same thing for the PEPs repo: python/peps#3791