Add isort pre-commit hook

Sort import statement in python files

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

api/libretimeapi/models/authentication.py

@@ -1,11 +1,13 @@
 import hashlib
+
 from django.contrib import auth
 from django.contrib.auth.models import AbstractBaseUser, Permission
 from django.core.exceptions import PermissionDenied
 from django.db import models
 from libretimeapi.managers import UserManager
 from libretimeapi.permission_constants import GROUPS
-from .user_constants import USER_TYPES, ADMIN
+
+from .user_constants import ADMIN, USER_TYPES
 
 
 class LoginAttempt(models.Model):