In 0.28.1:
Python 3.6.1 (v3.6.1:69c0db5050, Mar 21 2017, 01:21:04)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

from google.cloud import pubsub
cl = pubsub.Client()
Traceback (most recent call last):
File "", line 1, in
AttributeError: module 'google.cloud.pubsub' has no attribute 'Client'

In 0.27:
Python 3.6.1 (v3.6.1:69c0db5050, Mar 21 2017, 01:21:04)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

from google.cloud import pubsub
cl = pubsub.Client()
quit()