Python 3.9: Deprecated asyncio.wait use in Server.wait_closed

[icgood]

Using the grpclib Server class in Python 3.9 causes this deprecation warning:

  /.../python3.9/site-packages/grpclib/server.py:732: DeprecationWarning: The explicit passing of coroutine objects to asyncio.wait() is deprecated since Python 3.8, and scheduled for removal in Python 3.11.
    await asyncio.wait({h.wait_closed() for h in self._handlers})