Replace string with str in rtypes.

Used the command:
ag -l 'rtype: string' | xargs sed -i .bak 's/rtype: string/rtype: str/g'

Based on this comment:
googleapis/google-cloud-python#2485 (comment)

`str` is a type, `string` is a module.

Author: tswast

google/cloud/pubsub/_helpers.py

@@ -45,7 +45,7 @@ def topic_name_from_path(path, project):
     :param project: The project associated with the request. It is
                     included for validation purposes.
 
-    :rtype: string
+    :rtype: str
     :returns: Topic name parsed from ``path``.
     :raises: :class:`ValueError` if the ``path`` is ill-formed or if
              the project from the ``path`` does not agree with the
@@ -64,7 +64,7 @@ def subscription_name_from_path(path, project):
     :param project: The project associated with the request. It is
                     included for validation purposes.
 
-    :rtype: string
+    :rtype: str
     :returns: subscription name parsed from ``path``.
     :raises: :class:`ValueError` if the ``path`` is ill-formed or if
              the project from the ``path`` does not agree with the

google/cloud/pubsub/connection.py

@@ -83,7 +83,7 @@ def build_api_url(self, path, query_params=None,
                             Typically you shouldn't provide this and instead
                             use the default for the library.
 
-        :rtype: string
+        :rtype: str
         :returns: The URL assembled from the pieces provided.
         """
         if api_base_url is None:

google/cloud/pubsub/iam.py

@@ -123,7 +123,7 @@ def user(email):
         :type email: string
         :param email: E-mail for this particular user.
 
-        :rtype: string
+        :rtype: str
         :returns: A member string corresponding to the given user.
         """
         return 'user:%s' % (email,)
@@ -135,7 +135,7 @@ def service_account(email):
         :type email: string
         :param email: E-mail for this particular service account.
 
-        :rtype: string
+        :rtype: str
         :returns: A member string corresponding to the given service account.
         """
         return 'serviceAccount:%s' % (email,)
@@ -147,7 +147,7 @@ def group(email):
         :type email: string
         :param email: An id or e-mail for this particular group.
 
-        :rtype: string
+        :rtype: str
         :returns: A member string corresponding to the given group.
         """
         return 'group:%s' % (email,)
@@ -159,7 +159,7 @@ def domain(domain):
         :type domain: string
         :param domain: The domain for this member.
 
-        :rtype: string
+        :rtype: str
         :returns: A member string corresponding to the given domain.
         """
         return 'domain:%s' % (domain,)
@@ -168,7 +168,7 @@ def domain(domain):
     def all_users():
         """Factory method for a member representing all users.
 
-        :rtype: string
+        :rtype: str
         :returns: A member string representing all users.
         """
         return 'allUsers'
@@ -177,7 +177,7 @@ def all_users():
     def authenticated_users():
         """Factory method for a member representing all authenticated users.
 
-        :rtype: string
+        :rtype: str
         :returns: A member string representing all authenticated users.
         """
         return 'allAuthenticatedUsers'

google/cloud/pubsub/message.py

@@ -71,7 +71,7 @@ def timestamp(self):
     def service_timestamp(self):
         """Return server-set timestamp.
 
-        :rtype: string
+        :rtype: str
         :returns: timestamp (in UTC timezone) in RFC 3339 format
         """
         return self._service_timestamp