Instead of relying on attr.asdict recursion, we do recursion ourselves. This
simplifies a lot of the inherited to_dict() methods.

Related D2082

Daniele Serafini authored 5 years ago and Stefano Zacchiroli committed 5 years ago

... from test_persistent_identifier.

Closes T1986

It is now possible to install swh-model without dulwich (and Click, FWIW).

Users who want to use "swh identify" should "pip install swh-model[cli]".

make mypy 0.730 pass cleanly again

Needed by the replayer in swh-journal.

It kind of matches the current state of the postgresql storage, which
does not support it.

But keep support for deserializing from str, like
swh-storage does.