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/_helpers.py

@@ -526,7 +526,7 @@ def _pb_timestamp_to_rfc3339(timestamp_pb):
     :type timestamp_pb: :class:`google.protobuf.timestamp_pb2.Timestamp`
     :param timestamp_pb: A Google returned timestamp protobuf.
 
-    :rtype: string
+    :rtype: str
     :returns: An RFC 3339 formatted timestamp string.
     """
     timestamp = _pb_timestamp_to_datetime(timestamp_pb)

google/cloud/connection.py

@@ -179,7 +179,7 @@ def build_api_url(cls, 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.
         """
         url = cls.API_URL_TEMPLATE.format(

google/cloud/credentials.py

@@ -220,7 +220,7 @@ def generate_signed_url(credentials, resource, expiration,
     :param generation: (Optional) A value that indicates which generation of
                        the resource to fetch.
 
-    :rtype: string
+    :rtype: str
     :returns: A signed URL you can use to access the resource
               until expiration.
     """

google/cloud/streaming/transfer.py

@@ -163,7 +163,7 @@ def stream(self):
     def url(self):
         """URL to / from which data is downloaded/uploaded.
 
-        :rtype: string
+        :rtype: str
         :returns: The URL where data is sent/received.
         """
         return self._url
@@ -318,7 +318,7 @@ def total_size(self):
     def encoding(self):
         """'Content-Encoding' used to transfer the file
 
-        :rtype: string or None
+        :rtype: str or None
         :returns: The encoding of the downloaded content.
         """
         return self._encoding
@@ -732,7 +732,7 @@ def complete(self):
     def mime_type(self):
         """MIMEtype of the file being uploaded.
 
-        :rtype: string
+        :rtype: str
         :returns: The mime-type of the upload.
         """
         return self._mime_type
@@ -750,7 +750,7 @@ def progress(self):
     def strategy(self):
         """Upload strategy to use
 
-        :rtype: string or None
+        :rtype: str or None
         :returns: The strategy used to upload the data.
         """
         return self._strategy
@@ -972,7 +972,7 @@ def _get_range_header(response):
         :type response: :class:`google.cloud.streaming.http_wrapper.Response`
         :param response: response to be queried
 
-        :rtype: string
+        :rtype: str
         :returns: The header used to determine the bytes range.
         """
         # NOTE: Per RFC 2616[1]/7233[2][3], 'Range' is a request header,