Jenkins only runs tests for py37

[b1quint]

The recent issue #112 raised back the problem of which Python versions are supported by DRAGONS. Up to very recently, I remember that Py36 was being supported but is this still true? And what about future python versions? We are not testing them. The tests only run on Py37.

The Unit Tests in Jenkins could run several Python versions since they are short. That would help to verify the installation and potential problems. Very recently, we removed the parallel {} clause from the Unit Tests stage. I suggest adding this clause again to the Jenkinsfile with test cases where we have tests for different python versions.