Add in a Python 3 hello world

eg/python3/futures/hello-world.hy

@@ -0,0 +1,12 @@
+(import-from concurrent.futures ThreadPoolExecutor as-completed)
+(import-from random randint)
+
+(import-from sh sleep)
+
+(defn task-to-do [] (sleep (randint 1 5)))
+
+
+(with-as (ThreadPoolExecutor 10) executor
+         (setf jobs (list-comp (.submit executor task-to-do) (x (range 0 10))))
+         (for (future (as-completed jobs))
+           (.result future)))