Update docutils requirement from <0.18 to <0.19 (#1508)

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

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

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

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

docs/readthedocs-requirements.txt

@@ -1 +1 @@
-docutils<0.18
+docutils<0.19