Fix all the things by fixing multiprocessing (#733)

* Always use spawn.

* Relax python version requirements.

* Update setup.py

Co-authored-by: 姜 天戩 Mike Tian-Jian Jiang <tmjiang@gmail.com>

Co-authored-by: 姜 天戩 Mike Tian-Jian Jiang <tmjiang@gmail.com>

Author: stephenbach