Remove dephell and replace it with poetry

[tkilias]

• Dephell is not maintained anymore, so we can't rely on it
• We now use a bit hacky method to generate the setup.py by building the package with poetry and extracting the setup.py from the package (as we did it in the https://github.com/exasol/integration-test-docker-environment)
• We used Dephell also for running tests, this is now done by poetry

Fix #9