FIX: Python3 transactions and sqlalchemy

When used together with transaction and sqlalchemy,
it results in TypeErrorTypeError: unorderable types: int() < str().
SQLAlchemy uses string as sortKey while this uses int as sortKey
and these just does not work together in python3.
Merge branch 'patch-1' of https://github.com/hleumas/repoze.sendmail into hleumas-patch-1
Add a test for the value returned by 'sortKey'.
Merge branches 'hleumas-patch-1' and 'master'