* 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

chore: make ListMixin._list_filters always present

Always create GetMixin/GetWithoutIdMixin._optional_get_attrs attribute
with a default value of tuple()

This way we don't need to use hasattr() and we will know the type of
the attribute.

Always create ListMixin._list_filters attribute with a default value
of tuple().

This way we don't need to use hasattr() and we will know the type of
the attribute.

Always create RESTObject._short_print_attrs with a default value of
None.

This way we don't need to use hasattr() and we will know the type of
the attribute.

Convert _create_attrs and _update_attrs to use a NamedTuple
(RequiredOptional) to help with code readability. Update all code to
use the NamedTuple.