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)
- [Commits](https://github.com/python-babel/babel/compare/v2.7.0...v2.9.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

docs/requirements.txt

@@ -14,7 +14,7 @@ alabaster==0.7.12
     # via sphinx
 attrs==19.1.0
     # via packaging
-babel==2.7.0
+babel==2.9.1
     # via sphinx
 certifi==2019.6.16
     # via requests
@@ -35,9 +35,7 @@ markupsafe==1.1.1
 packaging==19.1
     # via sphinx
 pygments==2.7.4
-    # via
+    # via sphinx
-    #   pygments-cl-repl
-    #   sphinx
 pyparsing==2.4.2
     # via packaging
 pytz==2019.2
@@ -48,12 +46,12 @@ six==1.12.0
     # via packaging
 snowballstemmer==1.9.0
     # via sphinx
-sphinx-bootstrap-theme==0.7.1
-    # via -r requirements.in
 sphinx==2.2.0
     # via
     #   -r requirements.in
     #   sphinxcontrib-cldomain
+sphinx-bootstrap-theme==0.7.1
+    # via -r requirements.in
 sphinxcontrib-applehelp==1.0.1
     # via sphinx
 sphinxcontrib-devhelp==1.0.1