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/viz_tests.py

@@ -15,9 +15,9 @@
 # specific language governing permissions and limitations
 # under the License.
 from datetime import datetime
+from unittest.mock import Mock, patch
 import uuid
 
-from mock import Mock, patch
 import pandas as pd
 
 from superset import app