Bump docutils from 0.16 to 0.18

Bumps [docutils](http://docutils.sourceforge.net/) from 0.16 to 0.18.

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

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,2 +1,2 @@
-docutils==0.16
+docutils==0.18
 sphinx<3.6.0,>=3.2.0