Fix several "no active event loop" warnings

* Go back to IOLoop(make_current=False) to avoid proactively starting
  the loop when the underlying asyncio system is not ready.
* Turn streamz/tests/test_core.py::test_timed_window_timedelta into a
  coroutine as the underlying event loop needs to be live.

Author: dwskoog

streamz/core.py

@@ -51,7 +51,7 @@ def get_io_loop(asynchronous=None):
             return client.loop
 
     if not _io_loops:
-        loop = IOLoop()
+        loop = IOLoop(make_current=False)
         thread = threading.Thread(target=loop.start)
         thread.daemon = True
         thread.start()

streamz/tests/test_core.py

@@ -444,7 +444,8 @@ def test_timed_window_metadata():
     ]
 
 
-def test_timed_window_timedelta(clean):  # noqa: F811
+@pytest.mark.asyncio
+async def test_timed_window_timedelta(clean):  # noqa: F811
     pytest.importorskip('pandas')
     source = Stream(asynchronous=True)
     a = source.timed_window('10ms')

streamz/utils_test.py

@@ -43,7 +43,7 @@ def double(x):
 
 @contextmanager
 def pristine_loop():
-    loop = IOLoop()
+    loop = IOLoop(make_current=False)
     try:
         yield loop
     finally: