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

repository_archive() returns 'bytes' not 'str'

https://docs.gitlab.com/ee/api/repositories.html#get-file-archive

Fixes: #1584

Stop requiring a `name` attribute for creating a Release, since a
release name has not been required since GitLab 12.5.

Support merge_ref on merge requests that returns commit of attempted
merge of the MR.

Signed-off-by: Matej Focko <mfocko@redhat.com>

feat(api): add group hooks

Release API now supports PUT.

feat(api): add support for creating/editing reviewers in project MRs