specify python version for pip install (#859)

To avoid error caused by multiple versions of python and pip

setup.py

@@ -21,7 +21,7 @@ class build_ext(_build_ext.build_ext):
     def run(self):
         """Install traci wheels."""
         subprocess.check_call(
-            ['pip', 'install',
+            ['python3','-m','pip', 'install',
              'https://akreidieh.s3.amazonaws.com/sumo/flow-0.4.0/'
              'sumotools-0.4.0-py3-none-any.whl'])