Bump babel from 2.5.1 to 2.9.1 in /docs

Bumps [babel](https://github.com/python-babel/babel) from 2.5.1 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@v2.5.1...v2.9.1)

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

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

docs/requirements.txt

@@ -4,7 +4,7 @@ arrow==0.10.0
 asn1crypto==0.24.0
 attrs==18.2.0
 autobahn==0.15.0
-Babel==2.5.1
+Babel==2.9.1
 backports.ssl-match-hostname==3.5.0.1
 bcrypt==3.1.0
 certifi==2017.11.5