Fix: DeprecationWarning in Python3.7

Ref: python/cpython#5460
Ref: https://bugs.python.org/issue25988

aiozmq/core.py

@@ -7,7 +7,8 @@
 import weakref
 import zmq
 
-from collections import deque, Iterable, namedtuple
+from collections import deque, namedtuple
+from collections.abc import Iterable
 
 from .interface import ZmqTransport, ZmqProtocol
 from .log import logger

aiozmq/rpc/pubsub.py

@@ -1,5 +1,5 @@
 import asyncio
-from collections import Iterable
+from collections.abc import Iterable
 from functools import partial
 
 import zmq

aiozmq/selector.py

@@ -1,6 +1,6 @@
 """ZMQ pooler for asyncio."""
 import math
-from collections import Mapping
+from collections.abc import Mapping
 from errno import EINTR
 
 from zmq import (ZMQError, POLLIN, POLLOUT, POLLERR,

aiozmq/util.py

@@ -1,4 +1,4 @@
-from collections import Set
+from collections.abc import Set
 
 
 class _EndpointsSet(Set):