Get rid of DeprecationWarning: Request.is_xhr is deprecated

[squeaky-pl]

[snip 200 times]
webhooks/test_gpush_calendar_notifications.py::test_event_update
webhooks/test_gpush_calendar_notifications.py::test_event_update
webhooks/test_gpush_calendar_notifications.py::test_event_update
  /opt/venv/local/lib/python2.7/site-packages/werkzeug/local.py:347: DeprecationWarning: Request.is_xhr is deprecated. Given that the X-Requested-With header is not a part of any spec, it is not reliable
    return getattr(self._get_current_object(), name)

Seems to be coming from one of the dependencies since I cannot find any is_xhr in the repo.