Merge pull request python-trio#1918 from python-trio/dependabot/pip/incremental-21.3.0

Author: dependabot-preview[bot]

docs-requirements.txt

@@ -30,7 +30,7 @@ imagesize==1.2.0
     # via sphinx
 immutables==0.15
     # via -r docs-requirements.in
-incremental==17.5.0
+incremental==21.3.0
     # via towncrier
 jinja2==2.11.3
     # via