tox: test dilation on py3, but not on py2

tox.ini

@@ -10,7 +10,7 @@ minversion = 2.4.0
 
 [testenv]
 usedevelop = True
-extras = dev
+extras = dev,dilate
 deps =
     pyflakes >= 1.2.3
 commands =
@@ -18,6 +18,8 @@ commands =
     wormhole --version
     python -m wormhole.test.run_trial {posargs:wormhole}
 
+[testenv:py27]
+extras = dev
 
 # on windows, trial is installed as venv/bin/trial.py, not .exe, but (at
 # least appveyor) adds .PY to $PATHEXT. So "trial wormhole" might work on