Update Sphinx versions.

[sobolevnrm]

Fixes #384.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dd36b30) 66.35% compared to head (f44d38f) 66.35%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #385   +/-   ##
=======================================
  Coverage   66.35%   66.35%           
=======================================
  Files          30       30           
  Lines        7796     7796           
=======================================
  Hits         5173     5173           
  Misses       2623     2623           

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[orbeckst]

Looks ok. I assume the docs build correctly.

(I am not a big fan of pinning to exact versions but I understand that getting the docs to build can be finicky and that having exact versions helps keeping the docs lower maintenance.)