Drop a python 2 exception management

Previously (when python 2 was supported) we introduced some specific
exception management to detect connections issues when a rabbitmq
cluster node disappearing.

The original issue was that a ConnectionRefusedError is thrown and not
managed by oslo.messaging to detect heartbeat issue and call ensure_connection
for switching the connection destination (cluster node).

`ConnectionRefusedError` is only a python 3 exception [2].

Now that we only support python 3 we don't need to continue to
wrap/emulate this kind of exceptions (ConnectionRefusedError) [2] and
so we can drop the python 2 compatibility code to only support python 3 code.

[1] 9d8b1430e5
[2] https://docs.python.org/3/library/exceptions.html#ConnectionRefusedError

Change-Id: I4c459d8c947dac213a1866c0d37e8f3d547aa82e

oslo_messaging/_drivers/impl_rabbit.py

@@ -975,13 +975,6 @@ class Connection(object):
     def _heartbeat_thread_job(self):
         """Thread that maintains inactive connections
         """
-        # NOTE(hberaud): Python2 doesn't have ConnectionRefusedError
-        # defined so to switch connections destination on failure
-        # with python2 and python3 we need to wrapp adapt connection refused
-        try:
-            ConnectRefuseError = ConnectionRefusedError
-        except NameError:
-            ConnectRefuseError = socket.error
 
         while not self._heartbeat_exit_event.is_set():
             with self._connection_lock.for_heartbeat():
@@ -1008,7 +1001,7 @@ class Connection(object):
                     # ensure_connection for switching the
                     # connection destination.
                     except (socket.timeout,
-                            ConnectRefuseError,
+                            ConnectionRefusedError,
                             OSError,
                             kombu.exceptions.OperationalError) as exc:
                         LOG.info("A recoverable connection/channel error "