⬆️ Bump blacken-docs from 1.8.0 to 1.9.1

Bumps [blacken-docs](https://github.com/asottile/blacken-docs) from 1.8.0 to 1.9.1.
- [Release notes](https://github.com/asottile/blacken-docs/releases)
- [Commits](adamchainz/blacken-docs@v1.8.0...v1.9.1)

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

Author: dependabot[bot]

requirements_dev.txt

@@ -1,6 +1,6 @@
 autopep8==1.5.4
 black==20.8b1
-blacken-docs==1.8.0
+blacken-docs==1.9.1
 pip==20.3.3
 pre-commit==2.9.3
 twine==3.2.0