Add a tox.ini and migrate to pytest
Related swh/infra/ci-cd/swh-jenkins-jobs#1247 (closed) Related swh-environment#1261 (closed)
Test Plan
tox
Note: Should fail on 1 test, as it depends on pypi's swh-model v0.0.27, but should depend on v0.0.28 (not tagged yet)
fallback: make test
Migrated from D575 (view on Phabricator)