Bump babel from 2.3.4 to 2.9.1

Bumps [babel](https://github.com/python-babel/babel) from 2.3.4 to 2.9.1.
- [Release notes](https://github.com/python-babel/babel/releases)
- [Changelog](https://github.com/python-babel/babel/blob/master/CHANGES)
- [Commits](python-babel/babel@2.3.4...v2.9.1)

---
updated-dependencies:
- dependency-name: babel
  dependency-type: indirect
...

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

Author: dependabot[bot]

dev-requirements.txt

@@ -8,7 +8,7 @@ alabaster==0.7.8
     # via sphinx
 argh==0.26.2
     # via watchdog
-babel==2.3.4
+babel==2.9.1
     # via sphinx
 bumpversion==0.5.3
     # via -r dev-requirements.in