Fix pylint error "expression-not-assigned" and remove check from the
disabled list.

And I personally think it is much more readable now and is less lines
of code.

  * Add type-hints to some of the files in gitlab/v4/objects/
  * Fix issues detected when adding type-hints
  * Changed mypy exclusion to explicitly list the 13 files that have
    not yet had type-hints added.

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

  * Add type-hints to gitlab/v4/cli.py
  * Add required type-hints to other files based on adding type-hints
    to gitlab/v4/cli.py

Code was using inspect.getmro() to replicate the functionality of the
built-in function issubclass()

Switch to using issubclass()

Fix the import ordering using isort.

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

chore: make Get.*Mixin._optional_get_attrs always present