x509,certdb: handle certificates as bytes
Certificates, both in PEM and DER format, should be handled as bytes
in Python 3.
https://pagure.io/freeipa/issue/4985
Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>
Reviewed-By: Martin Basti <mbasti@redhat.com>