Add Test runs for Python 3.7 and remove 3.4 (#5295)

* remove 3.4 from unit test runs

* add 3.7 to most packages. PubSub, Monitoring, BigQuery not enabled

* Fix #5292 by draining queue in a way compatible with SimpleQueue and Queue

Author: crwilcox

nox.py

@@ -54,7 +54,7 @@ def default(session):
 
 
 @nox.session
-@nox.parametrize('py', ['2.7', '3.4', '3.5', '3.6'])
+@nox.parametrize('py', ['2.7', '3.5', '3.6', '3.7'])
 def unit(session, py):
     """Run the unit test suite."""