Set meaningless 5 items to None in Cursor.description

According to PEP249:

> The first two items (name and type_code) are mandatory, the other five
> are optional and are set to None if no meaningful values can be
> provided.

https://www.python.org/dev/peps/pep-0249/#description

Author: takuti

integration_tests/test_dbapi.py

@@ -56,7 +56,7 @@ def test_select_query(presto_connection):
     row = rows[0]
     assert row[0] == 'test'
     assert row[2] == fixtures.PRESTO_VERSION
-    columns = dict(cur.description)
+    columns = dict([desc[:2] for desc in cur.description])
     assert columns['node_id'] == 'varchar'
     assert columns['http_uri'] == 'varchar'
     assert columns['node_version'] == 'varchar'

prestodb/dbapi.py

@@ -210,8 +210,9 @@ def description(self):
         if self._query.columns is None:
             return None
 
+        # [ (name, type_code, display_size, internal_size, precision, scale, null_ok) ]
         return [
-            (col['name'], col['type']) for col in self._query.columns
+            (col['name'], col['type'], None, None, None, None, None) for col in self._query.columns
         ]
 
     @property