Merge pull request #1930 from python-trio/dependabot/pip/urllib3-1.26.4

Author: dependabot-preview[bot]

docs-requirements.txt

@@ -81,5 +81,5 @@ toml==0.10.2
     # via towncrier
 towncrier==19.2.0
     # via -r docs-requirements.in
-urllib3==1.26.3
+urllib3==1.26.4
     # via requests