upgrade the version of sphinx-rtd-theme to fix "style is undefined"

[ceblanton]

Describe your changes

Our sphinx CI building suddenly stopped working with this error:

Reason: UndefinedError("'style' is undefined")

Others have found that the cause is a mismatch between sphinx and sphinx-rtd-theme, and is fixed by updating the latter.

readthedocs/sphinx_rtd_theme#1571

Issue ticket number and link (if applicable)

Checklist before requesting a review

• I ran my code
• I tried to make my code readable
• I tried to comment my code
• I wrote a new test, if applicable
• I wrote new instructions/documentation, if applicable
• I ran pytest and inspected it's output
• I ran pylint and attempted to implement some of it's feedback

[ceblanton]

#124

[ilaflott]

I think we should be staying consistent between pip and conda in a way that seems intentional and non-arbitrary. I prefer the current version-

          pip install sphinx renku-sphinx-theme sphinx-rtd-theme
          pip install --upgrade sphinx-rtd-theme

i wonder if the renku one can be omitted, and the --upgrade flag can be added elsewhere for even more brevity? I'll try it out.

          pip install --upgrade sphinx sphinx-rtd-theme

[ilaflott]

no one-liner :-( but consistent