Fix python shebangs

Some systems only provide a versionned `python3` executable.

python_apps/pypo/bin/airtime-liquidsoap

@@ -1,4 +1,5 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
+
 import runpy
 
 # Run the liquidsoap python module

python_apps/pypo/bin/airtime-playout

@@ -1,4 +1,5 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
+
 import runpy
 
 runpy.run_module("pypo", run_name="__main__")

python_apps/pypo/bin/pyponotify

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import traceback