Rename strptime format constant:

- Ref. correct RFC.
- Private
- Add '_MICROS' to indicate which format from the sheaf allowed by the RFC.

Addresses points 1 and 2 from:
#816 (comment).

and:

#816 (comment)

gcloud/pubsub/message.py

@@ -19,7 +19,7 @@
 
 import pytz
 
-RFC3369 = '%Y-%m-%dT%H:%M:%S.%fZ'
+_RFC3339_MICROS = '%Y-%m-%dT%H:%M:%S.%fZ'
 
 
 class Message(object):
@@ -54,13 +54,13 @@ def timestamp(self):
         """Return timestamp from attributes, if passed.
 
         :rtype: datetime
-        :returns: timestamp (in UTC timezone) parsed from RFC 3369 timestamp
+        :returns: timestamp (in UTC timezone) parsed from RFC 3339 timestamp
         :raises: ValueError if timestamp not in ``attributes``, or malformed
         """
         stamp = self.attributes.get('timestamp')
         if stamp is None:
             raise ValueError('No timestamp')
-        return datetime.datetime.strptime(stamp, RFC3369).replace(
+        return datetime.datetime.strptime(stamp, _RFC3339_MICROS).replace(
             tzinfo=pytz.UTC)
 
     @classmethod

gcloud/pubsub/test_message.py

@@ -91,13 +91,13 @@ def _to_fail():
 
     def test_timestamp_w_timestamp_in_attributes(self):
         from datetime import datetime
-        import pytz
+        from pytz import utc
+        from gcloud.pubsub.message import _RFC3339_MICROS
         DATA = b'DEADBEEF'
         MESSAGE_ID = b'12345'
         TIMESTAMP = '2015-04-10T18:42:27.131956Z'
-        RFC3369 = '%Y-%m-%dT%H:%M:%S.%fZ'
-        naive = datetime.strptime(TIMESTAMP, RFC3369)
-        timestamp = naive.replace(tzinfo=pytz.utc)
+        naive = datetime.strptime(TIMESTAMP, _RFC3339_MICROS)
+        timestamp = naive.replace(tzinfo=utc)
         ATTRS = {'timestamp': TIMESTAMP}
         message = self._makeOne(data=DATA, message_id=MESSAGE_ID,
                                 attributes=ATTRS)