PDDB helper scripts: aes_gcm_siv now available in Cryptography

When the PDDB was first implemented, AES-GCM-SIV was not available as a standard primitive in the Python Cryptography library.

It's now available, and pyaesni, a dependency upon which the stopgap AES-GCM-SIV implementation based on the RFC requires, is bit-rotting.

This issue is a reminder to update the backalyzer, pddbcommon, etc. tools to the now standardized libraries.

note this was already monkey-patch tested in a temporary environment and the standard library is mostly compatible with minor refactorings.