Bump babel from 2.3.4 to 2.9.1 in /docs

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: direct:production
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -1,5 +1,5 @@
 alabaster==0.7.12
-Babel==2.3.4
+Babel==2.9.1
 certifi==2018.11.29
 chardet==3.0.4
 docutils==0.14