Bump babel from 2.7.0 to 2.9.1 in /docs

Bumps [babel](https://github.com/python-babel/babel) from 2.7.0 to 2.9.1.
- [Release notes](https://github.com/python-babel/babel/releases)
- [Changelog](https://github.com/python-babel/babel/blob/master/CHANGES.rst)
- [Commits](python-babel/babel@v2.7.0...v2.9.1)

---
updated-dependencies:
- dependency-name: babel
  dependency-type: indirect
...

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