This can be used instead of 'user_ids'

See: https://docs.gitlab.com/ee/api/merge_request_approvals.html#create-project-level-rule

The `project.approvals.set_approvals()` method used the
`/projects/:id/approvers` end point. That end point was removed from
GitLab in the 13.11 release, on 2-Apr-2021 in commit
27dc2f2fe81249bbdc25f7bd8fe799752aac05e6 via merge commit
e482597a8cf1bae8e27abd6774b684fb90491835. It was deprecated on
19-Aug-2019.

See merge request:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57473

In GitLab 14.10 they added support to get a single merge request
approval rule [1]

Add support for it to ProjectMergeRequestApprovalRuleManager

[1] https://docs.gitlab.com/ee/api/merge_request_approvals.html#get-a-single-merge-request-level-rule

Enable the pylint "unused-argument" check and resolve issues it found.

  * Quite a few functions were accepting `**kwargs` but not then
    passing them on through to the next level. Now pass `**kwargs` to
    next level.
  * Other functions had no reason to accept `**kwargs`, so remove it
  * And a few other fixes.

Using the CLI the command:
   gitlab project-merge-request-approval-rule list --mr-iid 1 --project-id foo/bar

Would raise an exception. This was due to the fact that `_id_attr` and
`_repr_attr` were set for keys which are not returned in the response.

Add a unit test which shows the `repr` function now works. Before it
did not.

This is an EE feature so we can't functional test it.

Closes: #2065

Enable the pylint check: "attribute-defined-outside-init" and fix
errors detected.

By having `RequiredOptional` in the `gitlab.base` module it makes it
difficult with circular imports. Move it to the `gitlab.types`
module which has no dependencies on any other gitlab module.

Commit f0152dc3cc9a42aa4dc3c0014b4c29381e9b39d6 removed situation
where `get()` in a `GetWithoutIdMixin` based class could return `None`

Update the type-hints to no longer return `Optional` AKA `None`