Bump docutils from 0.17.1 to 0.18 (#634)

Bumps [docutils](http://docutils.sourceforge.net/) from 0.17.1 to 0.18.

---
updated-dependencies:
- dependency-name: docutils
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/ci.txt

@@ -1,5 +1,5 @@
 codecov==2.1.12
-docutils==0.17.1
+docutils==0.18
 tox==3.24.4
 tox-travis==0.12
 pygments==2.10.0