(core) Simple Python 3 compatibility changes

Summary: Changes that move towards python 3 compatibility that are easy to review without much thought

Test Plan: The tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2873

sandbox/grist/testutil.py

@@ -3,6 +3,8 @@ import math
 import os
 import re
 
+import six
+
 import actions
 import logger
 
@@ -103,7 +105,7 @@ def parse_test_sample(obj, samples={}):
     }
 
   data = {t: table_data_from_rows(t, data[0], data[1:])
-          for t, data in obj["DATA"].iteritems()}
+          for t, data in six.iteritems(obj["DATA"])}
   return {"SCHEMA": schema, "DATA": data}