Set the 'iids' values as type ListAttribute so it will pass the list
as a comma-separated string, instead of a list.

Add a functional test.

Closes: #1407

fix: add a check to ensure the MRO is correct

It's mostly an internal thing anyway and can be removed in 3.0.0

Add a check to ensure the MRO (Method Resolution Order) is correct for classes in
gitlab.v4.objects when doing type-checking.

An example of an incorrect definition:
    class ProjectPipeline(RESTObject, RefreshMixin, ObjectDeleteMixin):
                          ^^^^^^^^^^ This should be at the end.

Correct way would be:
    class ProjectPipeline(RefreshMixin, ObjectDeleteMixin, RESTObject):
                                      Correctly at the end ^^^^^^^^^^

Also fix classes which have the issue.

fix: correct ProjectFile.decode() documentation

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

ProjectFile.decode() returns 'bytes' and not 'str'.

Update the method's doc-string and add a type-hint.

ProjectFile.decode() returns the result of a call to
base64.b64decode()

The docs for that function state it returns 'bytes':
https://docs.python.org/3/library/base64.html#base64.b64decode

Fixes: #1403

While adding type-hints mypy flagged this as an issue. The third
argument to register_custom_action is supposed to be a tuple. It was
being passed as a string rather than a tuple of strings.