Update python-traitlets to 4.3.3 and remove python2 support
- Adds :attr:Application.loaded_config_files property for accessing config files that have been loaded and makes the property idempotent on multiple loads.
Signed-off-by: Algent Albrahimi <email@example.com>
Test Plan: Run a couple of scripts in spyder3.
Reviewers: Triage Team, JoshStrobl
Reviewed By: Triage Team, JoshStrobl
Differential Revision: https://dev.getsol.us/D9267