Convert our manager usage to be done via type annotations.

Now to define a manager to be used in a RESTObject subclass can simply
do:
      class ExampleClass(CRUDMixin, RESTObject):
          my_manager: MyManager

Any type-annotation that annotates it to be of type *Manager (with the
exception of RESTManager) will cause the manager to be created on the
object.

Sometimes we have errors where we don't get a dictionary passed to
RESTObject.__init__() method. This breaks things but in confusing
ways.

Check in the __init__() method and raise an exception if it occurs.

Move the 'gitlab/tests/' directory to 'tests/unit/' so we have all the
tests located under the 'tests/' directory.