This actually fixes the debian build failure.

Related to T3666

As it's exposed through the swh.scheduler.pytest_plugin itself used by other
swh modules, this needs to be declared to avoid other swh module build
failures.

Related to T2746

Defining the pytest-plugin though the pytest-plugin [1] makes it loaded by default.
This creates loading issues on modules depending on scheduler but not on
the pytest plugin scheduler exposes as explained in the doc [2]

Instead we'll explicitely define to modules depending on the pytest plugins in
their root conftest [3]:

pytest_plugins = [ "swh.scheduler.pytest_plugin" ]

[1] https://docs.pytest.org/en/stable/writing_plugins.html#setuptools-entry-points

[2] https://docs.pytest.org/en/stable/writing_plugins.html#plugin-discovery-order-at-tool-startup

[3] https://docs.pytest.org/en/stable/writing_plugins.html#requiring-loading-plugins-in-a-test-module-or-conftest-file

Related to D3475
Related to T2484

This got solved when we started using the shared_task decorator instead of
instantiating our own app.

Related D2109
Related D2110

This reverts commit 3939a41d.

Wasn't supposed to be committed yet.

Sphinx complains about it.