Update samples to support latest Google Cloud Python [(googleapis#656)](

GoogleCloudPlatform/python-docs-samples#656)

samples/snippets/publisher.py

@@ -30,15 +30,7 @@ def list_topics():
     """Lists all Pub/Sub topics in the current project."""
     pubsub_client = pubsub.Client()
 
-    topics = []
-    next_page_token = None
-    while True:
-        page, next_page_token = pubsub_client.list_topics()
-        topics.extend(page)
-        if not next_page_token:
-            break
-
-    for topic in topics:
+    for topic in pubsub_client.list_topics():
         print(topic.name)
 
 

samples/snippets/requirements.txt

@@ -1 +1 @@
-google-cloud-pubsub==0.20.0
+google-cloud-pubsub==0.21.0

samples/snippets/subscriber.py

@@ -31,15 +31,7 @@ def list_subscriptions(topic_name):
     pubsub_client = pubsub.Client()
     topic = pubsub_client.topic(topic_name)
 
-    subscriptions = []
-    next_page_token = None
-    while True:
-        page, next_page_token = topic.list_subscriptions()
-        subscriptions.extend(page)
-        if not next_page_token:
-            break
-
-    for subscription in subscriptions:
+    for subscription in topic.list_subscriptions():
         print(subscription.name)