Bump babel from 2.4.0 to 2.9.1 in /docs (#48)

Author: dependabot[bot]

docs/requirements.txt

@@ -4,7 +4,7 @@ Jinja2==2.11.3
 MarkupSafe==1.1.1
 Pygments==2.7.4
 alabaster==0.7.10
-babel==2.4.0
+babel==2.9.1
 gitdb==0.6.4
 imagesize==0.7.1
 pytz==2017.2