Add return type hints to `deferred_gettext` variants
Add return type hints to `deferred_gettext` variants

Even though the return types are obvious, this is at least necessary
for pycharm to actually emit a warning when there is a signature
mismatch when using e.g. a `SimpleMessage` method incorrectly.

Refs #556

agdsn/pycroft,agdsn/pycroft,agdsn/pycroft,agdsn/pycroft,agdsn/pycroft