Use --upgrade instead of --force-reinstall for pip installs

[chrisjsewell]

As discussed in #5545, the --force-reinstall causes a lot of issues for Conda builds,
whereby some packages (for example docutils) cannot be uninstalled by pip.

A more comprehensive solution may be to include additional options in the .readthedocs.yaml,
to control the flags used when pip installing.
But, at least as an intermediary solution, I see no drawbacks in using --upgrade --upgrade-strategy eager to achieve the same objective, of ensuring dependencies are up-to-date (but without unnecessary uninstall/reinstall of dependencies).

[humitos]

I didn't read the whole thread about the problems.

That said, in case this got merged at some point, it should be under a project feature flag since it has a lot of impact (all projects installing their own package). As we cannot test them all and we had have some random problems on custom projects depending their needings, I prefer to go more defensive here instead.

So, from my perspective we need:

• wrap this into a feature flag
• update the docs mentioning this flag
• maybe, write a user guide explaining in what situations this is needed to activate/request

[stsewd]

I'm more at not using pip when using coda as their docs recomend https://www.anaconda.com/using-pip-in-a-conda-environment/ (we need to change our code to install the rtd deps from conda)

[humitos]

I'm not sure, but I think we can't install the package from the repository using conda (is it possible to do conda install . or similar?)

I don't have an strong knowledge on conda and if "just using it" will solve all these issues, but not relying on pip for projects using conda will be a much bigger task. On the other hand, we will need to publish our own extensions to conda-forge as well, not only PyPI.

[ericholscher]

This should make conda builds work better, so I'm 👍