currently provides mainly 2 generators:
- gen_origins()
- gen_contents()

we do not really need them to be mutable, plus we gain their instances now
being hashable, so we can add them in set() for example.

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.