Remove slots from SecurityOptions
Python 3 doesn't support slots and class variables with the same name
Replace DictMixin with MutableMapping under Py3
Remove the L suffix from long literals
No longer needed since Python 2.2
Fix StringIO import errors under Python 3
Fix error when adding integers > ffffffff to a Message
Fix pkey loading when running under Python 3
Add six as a requirement
Made exception catching compatible with Py3
Work around the lack of tuple parameter unpacking in Py3
Replace octal literals with decimal literals