Add documentation to util function.

biscuit/core/util.py

 import pkgutil
+from typing import Sequence
+
+def get_app_packages() -> Sequence:
+    """ Find all packages within the biscuit.apps namespace. """
 
-def get_app_packages():
     import biscuit.apps
     pkgs = ['biscuit.apps.%s' % i[1] for i in pkgutil.iter_modules(biscuit.apps.__path__)]