Pin docutils to <0.18

[bachya]

Describe what the PR does:

A recent docutils update broke our Read The Docs. One of their members suggested suggested pinning docutils to be <0.18, so this PR accomplishes that.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md and docs/ with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #72 (e5c9c57) into dev (081e388) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev       #72   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          291       291           
=========================================
  Hits           291       291           

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