travis should run tests against all supported database engines

[OriHoch]

• should use the travis Build Matrix feature to run the tests on all supported database engines
• I guess it means sqlite and PostgreSQL, but maybe we could add mysql as well?
• on each build it should:
  • setup the DB engine (perhaps using docker)
  • set the OVERRIDE_TEST_DB environment variable accordingly
  • run the tests, passing this environment variable to tox environment

[OriHoch]

another possibility - use docker compose yaml files to setup the test environment for each db engine