Bump babel from 2.5.3 to 2.9.1 in /docs/source

Bumps [babel](https://github.com/python-babel/babel) from 2.5.3 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@v2.5.3...v2.9.1)

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

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

docs/source/requirements.txt

@@ -1,6 +1,6 @@
 alabaster==0.7.10
 argh==0.26.2
-Babel==2.5.3
+Babel==2.9.1
 backports-abc==0.5
 certifi==2017.11.5
 chardet==3.0.4