tests: Simplify setup after pytest 8.1 release

Since the release of pytest 8.1, some pytest options are no longer needed and editable install can be used when running tests using tox.

I also added a commit to prevent sentry to send real HTTP requests when running tests (see https://jenkins.softwareheritage.org/job/DENV/job/tests/80/artifact/swh-loader-core.log).