Deal with removed AbstractChildWatcher

Fixes #637

Author: graingert

uvloop/includes/stdlib.pxi

@@ -44,7 +44,7 @@ cdef aio_isfuture = getattr(asyncio, 'isfuture', None)
 cdef aio_get_running_loop = getattr(asyncio, '_get_running_loop', None)
 cdef aio_set_running_loop = getattr(asyncio, '_set_running_loop', None)
 cdef aio_debug_wrapper = getattr(asyncio.coroutines, 'debug_wrapper', None)
-cdef aio_AbstractChildWatcher = asyncio.AbstractChildWatcher
+cdef aio_AbstractChildWatcher = getattr(asyncio, 'AbstractChildWatcher', object())
 cdef aio_Transport = asyncio.Transport
 cdef aio_FlowControlMixin = asyncio.transports._FlowControlMixin