use === instead of isa, it can be cheaper AFAIK

Author: nsajko

base/task.jl

@@ -1205,7 +1205,7 @@ function wait()
     have_result = false
     W = workqueue_for(Threads.threadid())
     task = trypoptask(W)
-    if !(task isa Task)
+    if task === nothing
         # No tasks to run; switch to the scheduler task to run the
         # thread sleep logic.
         sched_task = get_sched_task()