fix CBT computing when cert digest algo is not MD5, SHA1 or SHA256

[ThePirateWhoSmellsOfSunflowers]

This PR fixes the CBT computing when cert digest is not MD5, SHA1 ou SHA256 (see this comment). It also adds a "new" dependency: cryptography, however it seems that the project already uses it within the kerberos file but without installing it.

I choose cryptography because the project includes it already and because I havent found builtin python library that can parse a x509 certificate.

:sunflower: