The addition of '-p no:pytest_swh_core' in pytest.ini is needed to
prevent pytest from loading the pytest_swh_core plugin which we do not
need here and which would require some more dependencies (e.g.
requests).

This allows the `swh identify` command to work again.

Close T2288

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]".