Pylint fixes for datastore.

Removing an unused import and also adding a class-level
attribute for ``next_page_token`` which gets set outside of the
constructor (Pylint seems to miss the fact that it gets set
in the constructor of the base class).

Author: dhermes

datastore/google/cloud/datastore/query.py

@@ -397,6 +397,8 @@ class Iterator(BaseIterator):
                        query results.
     """
 
+    next_page_token = None
+
     def __init__(self, query, client, limit=None, offset=None,
                  start_cursor=None, end_cursor=None):
         super(Iterator, self).__init__(

datastore/unit_tests/test_query.py

@@ -468,8 +468,6 @@ def test__process_query_results_done(self):
         self.assertFalse(iterator._more_results)
 
     def test__process_query_results_bad_enum(self):
-        from google.cloud.datastore._generated import query_pb2
-
         iterator = self._makeOne(None, None)
         more_results_enum = 999
         with self.assertRaises(ValueError):