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 <[email protected]>

Co-authored-by: 姜 天戩 Mike Tian-Jian Jiang <[email protected]>

Author: stephenbach