(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/records.py

@@ -97,6 +97,8 @@ class Record(object):
   def __nonzero__(self):
     return bool(self._row_id)
 
+  __bool__ = __nonzero__
+
   def __repr__(self):
     return "%s[%s]" % (self._table.table_id, self._row_id)
 
@@ -144,6 +146,8 @@ class RecordSet(object):
   def __nonzero__(self):
     return bool(self._row_ids)
 
+  __bool__ = __nonzero__
+
   def __eq__(self, other):
     return (isinstance(other, RecordSet) and
         (self._table, self._row_ids, self._group_by, self._sort_by) ==