Add support for `projects.groups.list()` endpoint.

Closes #1717

Add support for merge trains

There are small typos in:
- docs/gl_objects/deploy_tokens.rst
- gitlab/base.py
- gitlab/mixins.py
- gitlab/v4/objects/features.py
- gitlab/v4/objects/groups.py
- gitlab/v4/objects/packages.py
- gitlab/v4/objects/projects.py
- gitlab/v4/objects/sidekiq.py
- gitlab/v4/objects/todos.py

Fixes:
- Should read `treatment` rather than `reatment`.
- Should read `transferred` rather than `transfered`.
- Should read `registered` rather than `registred`.
- Should read `occurred` rather than `occured`.
- Should read `overridden` rather than `overriden`.
- Should read `marked` rather than `maked`.
- Should read `instantiate` rather than `instanciate`.
- Should read `function` rather than `fonction`.

BREAKING CHANGE: remove deprecated project.issuesstatistics
in favor of project.issues_statistics

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.

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

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