Bump babel from 2.8.0 to 2.9.1 in /bdsg/docs

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] <[email protected]>

Author: dependabot[bot]

bdsg/docs/requirements.txt

@@ -1,5 +1,5 @@
 alabaster==0.7.12
-Babel==2.8.0
+Babel==2.9.1
 breathe==4.34.0
 certifi==2020.6.20
 chardet==3.0.4