Adapt the python port to support python 3

Pretty straightforward; i did some optimizations in the `pkcs7` module (unneeded conversions from int to string and back to int again).

The `six` dependency is of course not really required, but this code is only an example implementation and should be adapted / copy-pasted in other projects, so not really a problem.

Arthur

Edit: I just saw the other python 3 pull request… I ported some of the proposed improvements in my commit.