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] <support@github.com>

docs/requirements.txt

@@ -2,7 +2,7 @@
 python-markdown-math==0.2
 
 alabaster==0.7.8
-Babel==2.3.4
+Babel==2.9.1
 docutils==0.12
 imagesize==0.7.1
 Jinja2==2.11.3