uasyncio: fix loop.cancel()

Schedule task for execution when pend_throw() returns None (and not False).
None is the default value returned by pend_throw() when called the first time.

Author: dbrignoli

uasyncio.core/uasyncio/core.py

@@ -76,7 +76,7 @@ def cancel(self, coro, exc = CancelledError):
                 prev = coro.pend_throw(exc)
                 if __debug__ and DEBUG:
                     log.debug("Cancelling %s asynchronously", coro)
-                if prev is False:
+                if prev is None:
                     _event_loop.remove_polled_cb(coro)
                     _event_loop.call_soon(coro)
             except TypeError: