This fixes tests hang when building package for debian buster.

Related to T4232

Legacy Lister classes from the swh.lister.core mdule are no longer
used in swh-lister codebase so it is time to remove them.

Also remove lister CLI options related to legacy Lister API.

As a consequence, the following requirements are no longer needed:
arrow, SQLAlchemy, sqlalchemy-stubs and testing.postgresql.

Closes T2442

This is no longer useful, as removing it and tests are still ok.

Then drop all unneeded and indirect imports

This allows us to actually run the lister instantiation code instead of relying
on the underlying structure of the lister object. In turn, this allows future
listers to use the scheduler right in their __init__.

Related T2032

in order to be able to run unit tests using celery pytest fixtures, we
use a dedicated swh_app fixture that ensure the "main" celery app
is the test app (otherwise subtasks won't work).