uasyncio: On scheduling ready coro, unmark it as I/O-waiting.

Coros which are passed to .add_reader()/.add_writer() are marked as
I/O-bound using .pend_throw(False). Before scheduling it for normal
execution again, we need to unmark it with .pend_throw(None).

uasyncio/uasyncio/__init__.py

@@ -85,6 +85,7 @@ class PollEventLoop(EventLoop):
                 if isinstance(cb, tuple):
                     cb[0](*cb[1])
                 else:
+                    cb.pend_throw(None)
                     self.call_soon(cb)