Allow those old (ZODB 3.2) dotted paths to work again.

Fallout:  testPersistentMapping.py hasn't actually run anything
for a long time, due to a mysterious "return None" at the start
of its test_suite() function.  Removed that.  Then its
checkNewPicklesAreSafe test failed.  Spent 15 minutes on that,
and gave up -- I'm still not even sure what it's _trying_ to
test.  Changed it to a TODO.