consider updating docutils version dependency

[ctb]

sphinx_rtd_theme has been fixed, per readthedocs/sphinx_rtd_theme#127; might be good to revisit #2139 (comment) and do some experimenting, once a new sphinx_rtd_theme is released.

The specific change that was introduced to fix this problem was here,

5cfde22

specifically, limiting docutils to < 0.18:

  "docutils>=0.17.1,<0.18",

[ctb]

(whoops, and I checked, and that's already been done in this PR ;)

% pip list | grep -i docutils
docutils                      0.20.1