(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/test_gencode.py

@@ -2,6 +2,8 @@ import unittest
 import difflib
 import re
 
+from six.moves import xrange
+
 import gencode
 import identifiers
 import records