Support --upgrade flag for pip installs

[wmayner]

Details

I'm using my own fork of sphinx_rtd_theme, but since the upstream version has already been installed by the time the project-specific requirements are installed, my fork is ignored because pip thinks that sphinx_rtd_theme is up-to-date.

This could be fixed by:

1. Keeping my fork's version higher than the upstream;
2. Giving it a different name;
3. Putting my fork directly in the project repository via e.g. git subtree;
4. Passing the --upgrade flag to the project-specific pip install.

(1) seems like a hack; (2) is also a hack; (3) is inelegant, since the theme already has a nice way of being required as a dependency, and subtrees are a pain anyway. So (4) seems like the best option, unless it would lead to problems on the RTD side.

Happy to submit a PR if this seems like a reasonable idea (pointing me to the relevant sections of the code would be helpful—I think it's readthedocs/doc_builder/python_environments.py?)

• Project URL: https://pyphi.readthedocs.io
• Build URL (if applicable): https://readthedocs.org/projects/pyphi/builds/5775564/
• Read the Docs username (if applicable): wmayner

[stsewd]

I think this was already solved here #3201

[stsewd]

This issue was already solved on #3201 (merged) as a continuation of #3077