use tox testenv extras to request testenv too

tox.ini

@@ -43,9 +43,14 @@ deps =
 # We add usedevelop=False because testing against a true installation gives
 # more useful results.
 usedevelop = False
-# We use extras=test to get things like "mock" that are required for our unit
-# tests.
-extras = test
+
+extras =
+       # Get general testing environment dependencies so we can run the tests
+       # how we like.
+       testenv
+
+       # And get all of the test suite's actual direct Python dependencies.
+       test
 
 setenv =
        # Define TEST_SUITE in the environment as an aid to constructing the