sphinx 4.1.0 (e.g. on readthedocs now) is failing to compile html

[teuben]

Recently readthedocs must have upgraded to sphinx 4.1.0, and it's now failing with:

 Could not import extension sphinx.builders.epub3 (exception: cannot import name 'RemovedInSphinx40Warning' from 'sphinx.deprecation'

which you can locally invoke via

  cd $NEMO/doc 
  pip install -U -r requirements.txt

after this "make html" fails in the source directory.

[teuben]

I sympathize with this comment:
https://samnicholls.net/2016/06/15/how-to-sphinx-readthedocs/
stuff like this makes me wonder if it was the right decision to convert to sphinx.

[teuben]

sphinx-doc/sphinx#9434 offers a local workaround

  pip install -U sphinxcontrib-htmlhelp
  pip install -U sphinxcontrib-serializinghtml

a fix on readthedocs itself will surely be coming soon.

[teuben]

Modified the requirements.txt to contain sphinx>=4.1.1 made it work again.