Hotfix doc workflow

[ilaflott]

Describe your changes

change the pip install line in .github/workflows/documentation.yml to no later than 7.3.7. leave other theme option comment in docs/conf.py to signal available option for themes.

Issue ticket number and link (if applicable)

https://github.com/NOAA-GFDL/fre-cli/issues/124 for tracking this, as this hotfix will likely break in the near-ish future.

Checklist before requesting a review

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

[ilaflott]

not a permanent fix. issue #125 for tracking how much of a headache doc building versions become