Test everything under python 3 in py36 tox env

Author: mvdbeek

planemo/virtualenv.py

@@ -9,7 +9,7 @@
 from galaxy.tools.deps.commands import which
 
 
-DEFAULT_PYTHON_VERSION = "2.7"
+DEFAULT_PYTHON_VERSION = os.environ.get("PLANEMO_DEFAULT_PYTHON_VERSION", "2.7")
 
 
 def create_and_exit(virtualenv_path, **kwds):

tox.ini

@@ -20,6 +20,7 @@ setenv =
     quick: PLANEMO_SKIP_GALAXY_TESTS=1
     py27: PLANEMO_SKIP_PYTHON3=1
     py36: PLANEMO_SKIP_PYTHON2=1
+    py36: PLANEMO_DEFAULT_PYTHON_VERSION=3
     gx: NOSE_ATTR=tests_galaxy_branch
     master: PLANEMO_TEST_GALAXY_BRANCH=master
     dev: PLANEMO_TEST_GALAXY_BRANCH=dev