Add isort pre-commit hook

Sort import statement in python files

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

api/libretimeapi/permission_constants.py

@@ -1,6 +1,8 @@
 import logging
+
 from django.contrib.auth.models import Group, Permission
-from .models.user_constants import GUEST, DJ, PROGRAM_MANAGER, USER_TYPES
+
+from .models.user_constants import DJ, GUEST, PROGRAM_MANAGER, USER_TYPES
 
 logger = logging.getLogger(__name__)