Bump docutils from 0.17.1 to 0.18.1 in /requirements

Bumps [docutils](http://docutils.sourceforge.net/) from 0.17.1 to 0.18.1.

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

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

requirements/docs.txt

@@ -21,9 +21,9 @@ Babel==2.9.1 \
     --hash=sha256:ab49e12b91d937cd11f0b67cb259a57ab4ad2b59ac7a3b41d6c06c0ac5b0def9 \
     --hash=sha256:bc0c176f9f6a994582230df350aa6e05ba2ebe4b3ac317eab29d9be5d2768da0
 # docutils is required by Sphinx
-docutils==0.17.1 \
-    --hash=sha256:cf316c8370a737a022b72b56874f6602acf974a37a9fba42ec2876387549fc61 \
-    --hash=sha256:686577d2e4c32380bb50cbb22f575ed742d58168cee37e99117a854bcd88f125
+docutils==0.18.1 \
+    --hash=sha256:23010f129180089fbcd3bc08cfefccb3b890b0050e1ca00c867036e9d161b98c \
+    --hash=sha256:679987caf361a7539d76e584cbeddc311e3aee937877c87346f31debc63e9d06
 # imagesize is required by Sphinx
 imagesize==1.3.0 \
     --hash=sha256:1db2f82529e53c3e929e8926a1fa9235aa82d0bd0c580359c67ec31b2fddaa8c \