use importlib instead of imp as it support python 3.5+ (#57160)
Summary:
Prevent some annoying deprecation warning when importing cpp_extensions
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57160
Reviewed By: astaff
Differential Revision: D28096751
Pulled By: albanD
fbshipit-source-id: f169ad4c4945b0fff54c0339052a29f95b9f1831
