Update python-isort to 5.5.1
Update python-isort to 5.5.1 and drop python2 support.
Changelog since version 4.3.21.
Test Plan: Used it to sort some python files successfully. Also rebuilt pylint against this version and verified its functionality.
Reviewers: Triage Team, JoshStrobl
Reviewed By: Triage Team, JoshStrobl
Differential Revision: https://dev.getsol.us/D9611