Bump babel from 2.2.0 to 2.9.1 in /doc

Bumps [babel](https://github.com/python-babel/babel) from 2.2.0 to 2.9.1.
- [Release notes](https://github.com/python-babel/babel/releases)
- [Changelog](https://github.com/python-babel/babel/blob/master/CHANGES)
- [Commits](python-babel/babel@2.2.0...v2.9.1)

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

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

Author: dependabot[bot]

doc/requirements.txt

@@ -1,6 +1,6 @@
 alabaster==0.7.7
 argh==0.26.1
-Babel==2.2.0
+Babel==2.9.1
 backports-abc==0.4
 backports.ssl-match-hostname==3.5.0.1
 certifi==2016.2.28