*.py: use proper '#!/usr/bin/env' shebang

Using hard-coded paths in shebang is a bad idea, because on different
systems Python interpreter can be installed in different places. See:

https://mail.python.org/pipermail/tutor/2007-June/054816.html

Change-Id: Ib729ece0c95254dc2b235f90eb731681df955bd1

gsm_call_fsm.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python2
 
 # Python implementation of GSM 04.08 call state machine for use with
 # OsmoNITB MNCC interface

mncc_sock.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python2
 
 # Python interface to OsmoNITB MNCC (Mobile Network Call Control)
 # interface

mncc_test.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python2
 
 # Python testing tool for establishing calls via the OsmoNITB MNCC
 # interface.

smpp_test.py

@@ -1,4 +1,5 @@
-#!/usr/bin/python
+#!/usr/bin/env python2
+
 import logging
 import sys