Add tox.ini file and migrate to pytest
Related swh/infra/ci-cd/swh-jenkins-jobs#1247 (closed) Related swh-environment#1261 (closed) Depends on swh-loader-core!7 (closed)
Test Plan
tox fails for now as it depends on swh-loader-core's diff swh-loader-core > v0.0.34 (which does not exist yet)
fallback: make test
Migrated from D576 (view on Phabricator)