Revert "py3: sortKey() must be a string as comparison between str and None/int/tuple now raises TypeError."

This reverts commit 192c2000

This seem to introduce a test failure with testCommitOrder (testInvalidationBug.TestInvalidationBug)

Traceback (most recent call last):
  File "/srv/slapgrid/slappart21/t/aai/soft/e55935f80d1e8e2072b08bb007b94a70/parts/erp5/Products/ERP5/tests/testInvalidationBug.py", line 93, in testCommitOrder
    self.assertEqual(result_list[1], [0,0])  # activity buffer first
AssertionError: Lists differ: [1, 0] != [0, 0]

First differing element 0:
1
0

- [1, 0]
?  ^

+ [0, 0]
?  ^