Ladok3

Bumps dependency `cryptography` to v3.1 to solve a problem of a missing
parameter `backend`.

Improves output when reporting results.

**Full Changelog**: https://github.com/dbosk/ladok3/compare/v1.17...v1.18

Improves error messages for the report command.

Improves the `report` command in bulk reporting mode. This syncs it with the
behaviour in the [intropy reporting tool](https://github.com/dbosk/intropy/tree/85dee9cbeb3671f129404884fdb7f530bbe4e63b/adm/reporting).

Changes the way the key is derived with which the cache is encrypted.
Previously we generated a key. Now we derive a key using PBKDF2 and the user's
LADOK credentials. This is a workaround for systems that don't have a keyring
available.

Bugfix for get_student_data_by_uid_JSON