Fixed BaseDefaultEventLoopPolicy issue.

Pending (https://github.com/MagicStack/uvloop/pull/638/files):

Removed: asyncio.AbstractChildWatcher, asyncio.SafeChildWatcher
In favor of: asyncio.create_subprocess_exec / asyncio.create_subprocess_shell

uvloop/loop.pyx:
2917 if sig == uv.SIGCHLD:
2918 if (hasattr(callback, 'self') and
2919: isinstance(callback.self, aio_AbstractChildWatcher)):
2920
2921 warnings_warn(

uvloop/includes/stdlib.pxi:
45 cdef aio_set_running_loop = getattr(asyncio, '_set_running_loop', None)
46 cdef aio_debug_wrapper = getattr(asyncio.coroutines, 'debug_wrapper', None)
47: cdef aio_AbstractChildWatcher = asyncio.AbstractChildWatcher
48 cdef aio_Transport = asyncio.Transport
49 cdef aio_FlowControlMixin = asyncio.transports._FlowControlMixin