Update hashbang to use /usr/bin/env python

Please use the /usr/bin/env python hashbang in order to properly detect alternative python installations (like those installed through homebrew on a mac).

helloworld.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 from trezorlib.client import TrezorClient
 from trezorlib.transport_hid import HidTransport