Add isort pre-commit hook

Sort import statement in python files

See https://github.com/PyCQA/isort

python_apps/pypo/liquidsoap/__main__.py

@@ -1,12 +1,14 @@
 """ Runs Airtime liquidsoap
 """
 import argparse
-import os
-from . import generate_liquidsoap_cfg
 import logging
+import os
 import subprocess
+
 from pypo import pure
 
+from . import generate_liquidsoap_cfg
+
 PYPO_HOME = "/var/tmp/airtime/pypo/"