Bump babel from 2.9.0 to 2.9.1 (#405)

Bumps [babel](https://github.com/python-babel/babel) from 2.9.0 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@v2.9.0...v2.9.1)

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

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requirements-lock.txt

@@ -11,7 +11,7 @@ aspy.yaml==1.3.0          # via pre-commit
 astroid==2.3.3            # via pylint
 async-generator==1.10     # via nbclient
 attrs==20.3.0             # via jsonschema, jupyter-cache, markdown-it-py
-babel==2.9.0              # via sphinx, sphinx-intl
+babel==2.9.1              # via sphinx, sphinx-intl
 bleach==3.3.0             # via nbconvert
 certifi==2020.11.8        # via requests
 cffi==1.14.4              # via argon2-cffi