Update sphinx-tabs from 1.1.13 to 1.2.1

Author: pyup-bot

docs/requirements.txt

@@ -1,7 +1,7 @@
 sphinx==1.8.5  # pyup: ignore
 sphinx-autoapi==1.4.0
 sphinx-rtd-theme==0.5.0
-sphinx-tabs==1.1.13
+sphinx-tabs==1.2.1
 sphinx-prompt==1.1.0
 sphinx-version-warning==1.1.2
 sphinx-notfound-page==0.5