Revert "py3: sortKey() must be a string as comparison between str and...

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

This reverts commit 192c2000.

This commit introduced a test failure with testCommitOrder
testInvalidationBug.TestInvalidationBug [1]

    Traceback (most recent call last):
      File "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]
    ?  ^

That commit was apparently in the good direction, because transaction
API specifies that sortKey must return string

https://github.com/zopefoundation/transaction/blob/6cf10102948a8f4e36092a9b402e3859d6db4ccd/CHANGES.rst#141-2013-02-20
https://github.com/zopefoundation/transaction/blob/33448417ab526327757f05e23b1884786c805629/src/transaction/interfaces.py#L524-L534

product/CMFActivity/ActivityBuffer.py

@@ -76,4 +76,4 @@ class ActivityBuffer(TM):
 
   def sortKey(self, *ignored):
     """Activities must be finished before databases commit transactions."""
-    return '-1'
+    return -1

product/CMFActivity/ActivityConnection.py

@@ -34,7 +34,7 @@ from Acquisition import aq_parent
 # If the sort order below doesn't work, we cannot guarantee the sort key
 # used below will actually result in the activity connection being committed
 # after the ZODB and Catalog data.
-assert '\x00' < '-1' < '0' < '1' < '\xff', "Cannot guarantee commit of activities comes after the appropriate data"
+assert None < 0 < '' < (), "Cannot guarantee commit of activities comes after the appropriate data"
 
 manage_addActivityConnectionForm = HTMLFile('dtml/connectionAdd', globals())
 
@@ -66,4 +66,4 @@ InitializeClass(ActivityConnection)
 
 class ActivityDB(DB):
 
-    _sort_key = '\xff'
+    _sort_key = (0,)

product/ERP5Type/TransactionalVariable.py

@@ -70,7 +70,7 @@ class TransactionalVariable(dict):
   _unregistered = True
 
   def sortKey(self):
-    return '\x00'
+    return None
 
   commit = tpc_vote = tpc_begin = tpc_abort = lambda self, transaction: None
 
@@ -127,7 +127,7 @@ class TransactionalResource(object):
     return not (args in tv or tv.add(args))
 
   def sortKey(self):
-    return '1'
+    return 1
 
   abort = commit = tpc_vote = tpc_begin = tpc_finish = tpc_abort = \
     lambda self, transaction: None