Support redis-py v2 and v3

Further to #946 this fixes the underlying issue in a easy-to-upgrade way
for end users, many of whom will have Redis installed via other means.
By having this check here and supporting both versions concurrently it
makes it easier for end users, and to use celery/kombu in projects that
use Redis elsewhere.

With this change it is possibly worth reverting #946

kombu/transport/redis.py

@@ -146,8 +146,15 @@ def __init__(self, *args, **kwargs):
     def append(self, message, delivery_tag):
         delivery = message.delivery_info
         EX, RK = delivery['exchange'], delivery['routing_key']
+        # TODO: Remove this once we soley on Redis-py 3.0.0+
+        if redis.VERSION[0] >= 3:
+            # Redis-py changed the format of zadd args in v3.0.0
+            zadd_args = [{time(): delivery_tag}]
+        else:
+            zadd_args = [time(), delivery_tag]
+
         with self.pipe_or_acquire() as pipe:
-            pipe.zadd(self.unacked_index_key, time(), delivery_tag) \
+            pipe.zadd(self.unacked_index_key, *zadd_args) \
                 .hset(self.unacked_key, delivery_tag,
                       dumps([message._raw, EX, RK])) \
                 .execute()