Be sure to use python3 in the envs created by managetests

managetests/branches.py

@@ -92,7 +92,7 @@ class Branch:
             "git --git-dir=%s/.git --work-tree=%s checkout -f %s" %
                 (self.manager.git_dir, self.manager.build_dir, self.name),
             "cd %s && invoke build" % self.manager.build_dir,
-            "virtualenv %s/%s" % (self.manager.envs_dir, self.name),
+            "virtualenv -p python3 %s/%s" % (self.manager.envs_dir, self.name),
             "%s/%s/bin/pip install %s/build/packages/*.whl" %
                 (self.manager.envs_dir, self.name, self.manager.build_dir),
             ("ln -s %s/%s/lib/python3.%s/site-packages/uitwww/static "