* ci: remove py2 ci (#280)

(cherry picked from commit 2ac583cf)

* Remove all usages of six

* makefile: remove defunct python2 test command

* setup.py: bump version to 3.0.0

This change removes python2 support. Python 3.6 or up are now required.

There are no other breaking changes in this release.
Co-authored-by: Tao Feng <fengtao04@gmail.com>