Allow requirements file to be URI not just a file path

[Mno-hime]

What's the problem this feature will solve?

I maintain several software branches. Each has its own requirements.txt in the source tree. The requirements.txt file is meant to be the same across all branches, but sometimes we fail to backport changes from main to other branches, and the file has outdated Sphinx et al. versions.

Describe the solution you'd like

Allow python -> install -> -requirements in .readthedocs.yaml to be a URI instead of limiting it to file path.

Then it translates to python -m pip install -r $URI, which works in the command line.

Alternative solutions

I tried various escaping of the URI, but nothing worked.