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.

[codeclimate]

Code Climate has analyzed commit 83df0c5 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #280 (83df0c5) into dev (525c30a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #280   +/-   ##
=======================================
  Coverage   99.77%   99.77%           
=======================================
  Files          13       13           
  Lines         893      893           
=======================================
  Hits          891      891           
  Misses          2        2           

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