Key rotation support

[komarov]

Hi!

Thank you for implementing this library and making it easier to use libsodium with django.

I am wondering if you considered implementing key rotation support?
E.g. in the same way as django-fernet-fields do it by supporting a list of keys.
Maybe there are some non-obvious complications with that that you have already evaluated.

Would you be potentially open to reviewing and incorporating a PR that would implement this?