Support docutils 0.18

[mcmtroffaes]

Also see #302

[codecov]

Codecov Report

Merging #304 (66d1e38) into develop (65153f7) will increase coverage by 0.94%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           develop      #304      +/-   ##
============================================
+ Coverage    99.05%   100.00%   +0.94%     
============================================
  Files           52        52              
  Lines         2435      2442       +7     
  Branches       299       430     +131     
============================================
+ Hits          2412      2442      +30     
+ Misses          23         0      -23     

Impacted Files	Coverage Δ
test/test_bibliography.py	100.00% <ø> (ø)
test/common.py	100.00% <100.00%> (ø)
test/test_numpydoc.py	100.00% <0.00%> (+82.14%)	⬆️

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