docs: fix readthedocs build

[cdriehuys]

Fixes #153.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8ec4607) to head (621b927).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #154   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          354       354           
  Branches        14        14           
=========================================
  Hits           354       354           

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