Move libtorch to py3 and cleanup other CircleCI config (#35700)
Summary:
This moves libtorch to Python 3.6 and cleans up other CircleCI config for the removal of python2.
Going to see if all tests pass on this and will also land before https://github.com/pytorch/pytorch/pull/35677
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35700
Differential Revision: D20767830
Pulled By: orionr
fbshipit-source-id: 0d5a8224b65829cc2b08a5844707e0c0e079421a
