* API changes covered:
* keys and interface modules removed
* SSlibSigner removed
* CryptoSigner added: this replaces the removed functionality
* DSSE "signatures" container type changed
* Currently pins a securesystemslib main branch commit:
this shoudl be reverted before merging, when securesystemslib
has made a release
* tests/generated_data/generate_md.py was simplified
* Encrypted test keys in tests/repository_data/keystore were replaced
with the unencrypted PEM versions of the same keys
* The public test keys in tests/repository_data/keystore were removed
as they were not used anymore
Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>
