Update docutils requirement from <0.18,>=0.16 to >=0.16,<0.21 (#4549)

Updates the requirements on [docutils](https://docutils.sourceforge.io/) to permit the latest version.

---
updated-dependencies:
- dependency-name: docutils
  dependency-type: direct:development
...

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

requirements-dev.txt

@@ -23,6 +23,6 @@ autoflake
 isort
 diff_cover
 lxml
-docutils>=0.16,<0.18
+docutils>=0.16,<0.21
 pyupgrade
 pytest-xdist