manually specify multiprocessing use fork in setup.py (#754)

Author: IanDelbridge

setup.py

@@ -45,6 +45,8 @@
 nthreads = mp.cpu_count()
 if os.name == "nt":
     nthreads = 0
+else:
+    mp.set_start_method("fork", force=True)
 
 setup(
     packages=packages,