Bump babel from 2.9.0 to 2.9.1 in /docs (#171)

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: indirect
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -8,9 +8,9 @@ alabaster==0.7.12 \
     --hash=sha256:446438bdcca0e05bd45ea2de1668c1d9b032e1a9154c2c259092d77031ddd359 \
     --hash=sha256:a661d72d58e6ea8a57f7a86e37d86716863ee5e92788398526d58b26a4e4dc02
     # via sphinx
-babel==2.9.0 \
-    --hash=sha256:9d35c22fcc79893c3ecc85ac4a56cde1ecf3f19c540bba0922308a6c06ca6fa5 \
-    --hash=sha256:da031ab54472314f210b0adcff1588ee5d1d1d0ba4dbd07b94dba82bde791e05
+babel==2.9.1 \
+    --hash=sha256:ab49e12b91d937cd11f0b67cb259a57ab4ad2b59ac7a3b41d6c06c0ac5b0def9 \
+    --hash=sha256:bc0c176f9f6a994582230df350aa6e05ba2ebe4b3ac317eab29d9be5d2768da0
     # via sphinx
 certifi==2020.12.5 \
     --hash=sha256:1a4995114262bffbc2413b159f2a1a480c969de6e6eb13ee966d470af86af59c \