Bump babel from 2.8.0 to 2.9.0

Bumps [babel](https://github.com/python-babel/babel) from 2.8.0 to 2.9.0.
- [Release notes](https://github.com/python-babel/babel/releases)
- [Changelog](https://github.com/python-babel/babel/blob/master/CHANGES)
- [Commits](https://github.com/python-babel/babel/commits)

sphinx-doc/requirements_sphinx.txt

 alabaster==0.7.12
 appdirs==1.4.4
 astor==0.8.1
+Babel==2.9.0
 attrs==20.3.0
-Babel==2.8.0
 black==20.8b1
 certifi==2020.11.8
 chardet==3.0.4