Bump babel from 2.8.0 to 2.9.1 in /doc/.sphinx

Bumps [babel](https://github.com/python-babel/babel) from 2.8.0 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@v2.8.0...v2.9.1)

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

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

doc/.sphinx/requirements.txt

@@ -6,7 +6,7 @@ testresources==2.0.1
 recommonmark==0.6
 ## The following requirements were added by pip freeze:
 alabaster==0.7.12
-Babel==2.8.0
+Babel==2.9.1
 certifi==2020.4.5.2
 chardet==3.0.4
 click==7.1.2