Bump incremental from 17.5.0 to 21.3.0

Bumps [incremental](https://github.com/twisted/incremental) from 17.5.0 to 21.3.0.
- [Release notes](https://github.com/twisted/incremental/releases)
- [Changelog](https://github.com/twisted/incremental/blob/master/NEWS.rst)
- [Commits](twisted/incremental@incremental-17.5.0...incremental-21.3.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

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