Py3: failure when data managers don't supply a sort key

[Chris McDonough]

In Python 3, when multiple datamanagers are joined to a transaction and those
datamanagers do not have a ``sortKey`` method, we would previously get an
exception in ``transaction._transaction.Transaction._commitResources``
since Python 3 no longer supports comparisons if the operands are None.

A patch is attached which causes ``transaction._transaction.rm_key`` to return the name of the datamanager
class instead of None if the datamanager does not have a ``sortKey`` method, which fixes this problem.

[Chris McDonough (chrism-plope)]

Note that this patch was originally supplied by Chris Rossi.