pytest_plugin: Update sentry fixtures after sentry-python 2.0 release
sentry-python 2.0 introduced some breaking API changes so update the sentry fixtures used in tests to restore previous behavior.
This fixes the new errors reported in https://jenkins.softwareheritage.org/job/DENV/job/tests/205/.
I also added a commit to allow initializing sentry with a None
DSN for testing purposes,
it will also enable to remove the use of a fake DSN when initializing sentry for celery tasks (see code).