Replace PyCrypto with Cryptography

[AdamIsrael]

It looks like PyCrypto is a dead project:

Compare, for example, https://github.com/dlitz/pycrypto/graphs/contributors?from=2015-06-15&to=2017-07-15&type=c vs. https://github.com/pyca/cryptography/graphs/contributors?from=2015-06-15&to=2017-07-15&type=c.

Many projects needing cryptography in Python, including (just off the top of my head) Twisted Conch and Paramiko, are migrating from PyCrypto to Cryptography.

I've been looking at Cryptography for another project. I want to verify if the Cryptography library is compatible with Python 2.7 and 3, and make sure it really does have the traction it seems to, because I'd hate for anyone, myself included, to put work into support of another crypto library only to have it fall out of favour shortly afterward.