This includes the work done by @bkabrda to port the crypto module to python3.
We found that:

• M2Crypto won't be ported to python3, so we need something else.
• python-cryptography looks nice.
• It can only do the message-signing part of what we need, not message validation.

So this pull request introduces an (ugly) split. If we're on py3, we use an
x509_ng module that uses cryptography for signing .. and will fail to validate
anything with an ImportError on M2Crypto. If we're on py2, then we use the
old standard x509 module.

I'd like to release this so we can try it with mailman3 which is python3 only.
We have a plugin for it
that only publishes (signs) messages, but does not receive (validate) them, so
we should be okay.