Fix classes which don't have correct 'get()' methods for classes
derived from GetMixin.

Add a unit test which verifies that classes have the correct return
type in their 'get()' method.

chore: add type-hints to gitlab/v4/objects/users.py

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.

Adding type-hints to gitlab/v4/objects/users.py

The 'managers' are dynamically created. This unfortunately means that
we don't have any type-hints for them and so editors which understand
type-hints won't know that they are valid attributes.

 * Add the type-hints for the managers we define.
 * Add a unit test that makes sure that the type-hints and the
   '_managers' attribute are kept in sync with each other.
 * Add unit test that makes sure specified managers in '_managers'
   have a name ending in 'Managers' to keep with current convention.
 * Make RESTObject._managers always present with a default value of
   None.
 * Fix a type-issue revealed now that mypy knows what the type is

Fix the import ordering using isort.

https://pycqa.github.io/isort/