Fix connection with non sqlite db

[PierrickBrun]

Fix https://github.com/lucafaggianelli/plombery/issues/363

This MR allows to connect to non-sqlite databases.

I ran the tests after the modification and they all passed.

I also ran the tests on an empty postgresql DB and they all passed except tests/test_logging.py::test_pipeline_logs_are_correclty_captured. All the logs appear in double, but the tasks are not doubled in the db.

The logs are not duplicated in the example app running on postgres too.

The same problem appears when I use sqlite as a file (opposed to an in-memory db). So I guess we can ignore this problem and consider postgres as working.