uasyncio.core: Add yield call to remove coro from scheduling.

yield False won't reschedule current coroutine to be run again. This is
useful when coro is put on some waiting queue (and is similar to what
yield IORead/yield IOWrite do).

uasyncio.core/uasyncio/core.py

@@ -116,6 +116,9 @@ class EventLoop:
                     elif ret is None:
                         # Just reschedule
                         pass
+                    elif ret is False:
+                        # Don't reschedule
+                        continue
                     else:
                         assert False, "Unsupported coroutine yield value: %r (of type %r)" % (ret, type(ret))
                 except StopIteration as e: