Bump babel from 2.7.0 to 2.9.1 in /docs

Bumps [babel](https://github.com/python-babel/babel) from 2.7.0 to 2.9.1.
- [Release notes](https://github.com/python-babel/babel/releases)
- [Changelog](https://github.com/python-babel/babel/blob/master/CHANGES)
- [Commits](https://github.com/python-babel/babel/compare/v2.7.0...v2.9.1)

---
updated-dependencies:
- dependency-name: babel
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -3,7 +3,7 @@ appdirs==1.4.3
 argh==0.26.2
 atomicwrites==1.3.0
 attrs==19.1.0
-Babel==2.7.0
+Babel==2.9.1
 black==18.9b0
 bleach==3.1.0
 certifi==2019.6.16