Update dependency requests to v2.21.0

Author: renovate-bot

requirements.txt

@@ -14,7 +14,7 @@ Mako==1.0.7
 MarkupSafe==1.0
 python-dateutil==2.6.1
 python-editor==1.0.3
-requests==2.18.4
+requests==2.21.0
 six==1.11.0
 SQLAlchemy==1.0.14
 urllib3==1.22