encoding should be part of str conversion and utf-8 is
the default anyway

python-cryptography needs newer rust package

It would simplify and speed up the code

Store CA foreign key instead of CA key identifier

To identify issuer of given cert straight from DB

Something goes wrong if duplicated serial_number is generated. The
process will fail on UNIQUE db constraint then.

Use AuthorityKeyIdentifier itself instead of extension in
from_issuer_subject_key_identifier classmethod

Some of them does not add any confidence level and some of them depends
on implementation details