Use cryptography's built-in PKCS7 padding functions

[tannercollin]

Using cryptography's built-in padding functions is more readable than manually appending / removing the bytes.

I've tested the changes:

I assign all copyright for the changes to you :)

[maqp]

Thank you so much! The 1.19.04 now uses pyca/cryptography's PKCS7 implementation. I apologize I'm not able to trivially merge pull requests. This is because all source files have pinned hashes in the installer, which is signed using an airgapped system. So any update in the source requires updating the installer and its signature at the same time. I've credited you in the update log, I hope that's enough!

[tannercollin]

Absolutely! Thanks for the update.