Injectable statsd client (#7138)

* Add ability to inject statsd client; some py test/reqs updates

- Updated the metrics logger to allow construction with an existing
statsd client, so that it can be configured by external systems or libs.
- added requirements to requirements-dev.txt which are needed to run
  tests-eg coverage, nose
- removed dependency on mock lib, it is in python stdlib now
- updated tox.ini to remove the now-superfluous deps

* add license to test file, and remove blank line at EOF

tests/utils_tests.py

@@ -17,9 +17,9 @@
 from datetime import date, datetime, time, timedelta
 from decimal import Decimal
 import unittest
+from unittest.mock import patch
 import uuid
 
-from mock import patch
 import numpy
 
 from superset.exceptions import SupersetException