Use cryptography instead of pyCrypto / oscrypto in document.

[honglei]

PyCrypto is not maintained, try use pycryptodome.

https://python-pkcs11.readthedocs.io/en/latest/opensc.html?highlight=store#id1

[danni]

Pull request welcome.

[MatthiasValvekens]

I changed the scope of this issue a bit, I'd get rid of both pyCrypto and oscrypto (same reason: lack of upstream maintenance) and refer to cryptography instead.