Bump python-dateutil from 2.6.1 to 2.8.1 (#1526)

Bumps [python-dateutil](https://github.com/dateutil/dateutil) from 2.6.1 to 2.8.1.
- [Release notes](https://github.com/dateutil/dateutil/releases)
- [Changelog](https://github.com/dateutil/dateutil/blob/master/NEWS)
- [Commits](dateutil/dateutil@2.6.1...2.8.1)

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base-requirements.txt

@@ -23,7 +23,7 @@ elasticsearch>=5,<6
 django-tastypie==0.14.1
 
 pytz==2017.2
-python-dateutil==2.6.1
+python-dateutil==2.8.1
 
 requests[security]>=2.20.0