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/datastore/client.py

@@ -53,7 +53,7 @@ def _determine_default_project(project=None):
     :type project: string
     :param project: Optional. The project to use as default.
 
-    :rtype: string or ``NoneType``
+    :rtype: str or ``NoneType``
     :returns: Default project if it can be determined.
     """
     if project is None:

google/cloud/datastore/connection.py

@@ -76,7 +76,7 @@ def _request(self, project, method, data):
         :param data: The data to send with the API call.
                      Typically this is a serialized Protobuf string.
 
-        :rtype: string
+        :rtype: str
         :returns: The string response content from the API call.
         :raises: :class:`google.cloud.exceptions.GoogleCloudError` if the
                  response code is not 200 OK.

google/cloud/datastore/key.py

@@ -269,7 +269,7 @@ def is_partial(self):
     def namespace(self):
         """Namespace getter.
 
-        :rtype: string
+        :rtype: str
         :returns: The namespace of the current key.
         """
         return self._namespace
@@ -298,7 +298,7 @@ def flat_path(self):
     def kind(self):
         """Kind getter. Based on the last element of path.
 
-        :rtype: string
+        :rtype: str
         :returns: The kind of the current key.
         """
         return self.path[-1]['kind']
@@ -316,7 +316,7 @@ def id(self):
     def name(self):
         """Name getter. Based on the last element of path.
 
-        :rtype: string
+        :rtype: str
         :returns: The (string) name of the key.
         """
         return self.path[-1].get('name')
@@ -335,7 +335,7 @@ def id_or_name(self):
     def project(self):
         """Project getter.
 
-        :rtype: string
+        :rtype: str
         :returns: The key's project.
         """
         return self._project
@@ -391,7 +391,7 @@ def _validate_project(project, parent):
     :type parent: :class:`google.cloud.datastore.key.Key` or ``NoneType``
     :param parent: The parent of the key or ``None``.
 
-    :rtype: string
+    :rtype: str
     :returns: The ``project`` passed in, or implied from the environment.
     :raises: :class:`ValueError` if ``project`` is ``None`` and no project
              can be inferred from the parent.

google/cloud/datastore/query.py

@@ -109,7 +109,7 @@ def project(self):
     def namespace(self):
         """This query's namespace
 
-        :rtype: string or None
+        :rtype: str or None
         :returns: the namespace assigned to this query
         """
         return self._namespace or self._client.namespace
@@ -128,7 +128,7 @@ def namespace(self, value):
     def kind(self):
         """Get the Kind of the Query.
 
-        :rtype: string
+        :rtype: str
         :returns: The kind for the query.
         """
         return self._kind

google/cloud/datastore/transaction.py

@@ -100,7 +100,7 @@ def __init__(self, client):
     def id(self):
         """Getter for the transaction ID.
 
-        :rtype: string
+        :rtype: str
         :returns: The ID of the current transaction.
         """
         return self._id