Change PyCrypto to cryptography

[roaldnefs]

Change PyCryto to cryptography because the PyCrypto library isn't updated in the last couple of years and has a known vulnerability.

The cryptography library is able to read the private key file in both PKCS and OpenSSH format.

See:

• pycrypto#253
• TElgamal/attack-on-pycrypto-elgamal
• pycrypto#275
• pycrypto#158