(core) Use MixedTypesKey for sort_by arg of lookupRecords to avoid errors in Python 3

Summary: title

Test Plan: Added python unit test. Seems like the first test of sort_by in the whole codebase.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D3124

sandbox/grist/grist.py

@@ -10,6 +10,7 @@ from usertypes import Any, Text, Blob, Int, Bool, Date, DateTime, \
 from usertypes import PositionNumber, ManualSortPos, Reference, ReferenceList, formulaType
 from table import UserTable
 from records import Record, RecordSet
+from column import SafeSortKey
 
 DOCS = [(__name__, (Record, RecordSet, UserTable)),
         ('lookup', (UserTable.lookupOne, UserTable.lookupRecords))]