Bump babel from 2.8.0 to 2.9.1 in /docs (#104)

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: indirect
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -8,7 +8,7 @@ alabaster==0.7.12
     # via sphinx
 argh==0.26.2
     # via sphinx-autobuild
-babel==2.8.0
+babel==2.9.1
     # via sphinx
 certifi==2020.6.20
     # via requests
@@ -54,17 +54,17 @@ six==1.15.0
     #   sphinxcontrib-napoleon
 snowballstemmer==2.0.0
     # via sphinx
-sphinx-autobuild==0.7.1
-    # via -r docs/requirements.in
-sphinx-autodoc-typehints==1.11.0
-    # via -r docs/requirements.in
-sphinx-rtd-theme==0.5.0
-    # via -r docs/requirements.in
 sphinx==3.1.2
     # via
-    #   -r docs/requirements.in
+    #   -r requirements.in
     #   sphinx-autodoc-typehints
     #   sphinx-rtd-theme
+sphinx-autobuild==0.7.1
+    # via -r requirements.in
+sphinx-autodoc-typehints==1.11.0
+    # via -r requirements.in
+sphinx-rtd-theme==0.5.0
+    # via -r requirements.in
 sphinxcontrib-applehelp==1.0.2
     # via sphinx
 sphinxcontrib-devhelp==1.0.2
@@ -74,7 +74,7 @@ sphinxcontrib-htmlhelp==1.0.3
 sphinxcontrib-jsmath==1.0.1
     # via sphinx
 sphinxcontrib-napoleon==0.7
-    # via -r docs/requirements.in
+    # via -r requirements.in
 sphinxcontrib-qthelp==1.0.3
     # via sphinx
 sphinxcontrib-serializinghtml==1.1.4