Fix unorderable types error (#4409)

If the updated datetime collides, the comparator of heapq will move
onto the AuthorizationResource value and throws an "unorderable type"
error.

This adds an index value to the element tuple to ensure that they are
always strictly ordered.

acme/acme/client.py

@@ -360,14 +360,18 @@ class Client(object):  # pylint: disable=too-many-instance-attributes
 
         # priority queue with datetime.datetime (based on Retry-After) as key,
         # and original Authorization Resource as value
-        waiting = [(datetime.datetime.now(), authzr) for authzr in authzrs]
+        waiting = [
+            (datetime.datetime.now(), index, authzr)
+            for index, authzr in enumerate(authzrs)
+        ]
+        heapq.heapify(waiting)
         # mapping between original Authorization Resource and the most
         # recently updated one
         updated = dict((authzr, authzr) for authzr in authzrs)
 
         while waiting:
             # find the smallest Retry-After, and sleep if necessary
-            when, authzr = heapq.heappop(waiting)
+            when, index, authzr = heapq.heappop(waiting)
             now = datetime.datetime.now()
             if when > now:
                 seconds = (when - now).seconds
@@ -386,7 +390,7 @@ class Client(object):  # pylint: disable=too-many-instance-attributes
                 if attempts[authzr] < max_attempts:
                     # push back to the priority queue, with updated retry_after
                     heapq.heappush(waiting, (self.retry_after(
-                        response, default=mintime), authzr))
+                        response, default=mintime), index, authzr))
                 else:
                     exhausted.add(authzr)