Fix the import ordering using isort.

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

chore: make Get.*Mixin._optional_get_attrs 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.

Fix all issues reported by running: tox -e pep8 and enable pep8 as a linter check

Commit baea7215bbbe07c06b2ca0f97a1d3d482668d887 introduced type-hints
for gitlab/mixins.py.

After starting to add type-hints to gitlab/v4/objects/users.py
discovered a few errors.

Main error was using '=' instead of ':'. For example:
  _parent = Optional[...] should be _parent: Optional[...]

Resolved those issues.

F401: Module imported but unused

https://www.flake8rules.com/rules/F401.html

chore: remove usage of getattr()