runtests.py: Use python3 explicitly.

Change the shebang for runtests.py from
python to python3 so that Python 3 is
always explicitly invoked, regardless of
the system default.

ASTERISK-30207 #close

Change-Id: Iaadc9e468d3060d419397f27f457be68b1b05d88

Author: InterLinked1

runtests.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 '''Asterisk external test suite driver.
 
 Copyright (C) 2015, Digium, Inc.