use python3.6 on linux but any python3 on windows
use python3.6 on linux but any python3 on windows

Reviewed By: kmancini

Differential Revision: D25890432

fbshipit-source-id: 6b216ed998e2f6169a47b9553e40a7abe611bcdd

facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman