Description of the problem, including code/CLI snippet

gitlab event list fails in v4/cli.py#l.38 with AttributeError: 'NoneType' object has no attribute 'lower'.

Specifications

• python-gitlab version: 1.8.0
• API version you are using (v3/v4): na
• Gitlab server version (or gitlab.com): na

Cause

The error is caused by the EventManager class in objects.py, because one of _list_filters is called "action". This name collides with the args attribute action, and one of them is probably overwritten. When renaming "action" in _list_filters to something else, gitlab event list continues further (and fails with another similar error).