(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/action_summary.py

@@ -4,6 +4,8 @@ It's used for collecting calculated values for formula columns.
 """
 from collections import namedtuple
 
+import six
+
 import actions
 from objtypes import equal_encoding
 
@@ -76,7 +78,7 @@ class ActionSummary(object):
     """
     if not column_delta:
       return
-    full_row_ids = sorted(r for r, (before, after) in column_delta.iteritems()
+    full_row_ids = sorted(r for r, (before, after) in six.iteritems(column_delta)
                           if not equal_encoding(before, after))
 
     defunct = is_defunct(table_id) or is_defunct(col_id)