Update python-traitlets to 4.3.3 and remove python2 support

Summary:
Changelog:

• 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 <algent@protonmail.com>

Test Plan: Run a couple of scripts in spyder3.

Reviewers: Triage Team, JoshStrobl

Reviewed By: Triage Team, JoshStrobl

Subscribers: JoshStrobl

Differential Revision: https://dev.getsol.us/D9267