Pulsar docker

This MR addresses the issue mentioned by @daniel-eggert in #23 (closed) and changes the way how we run the CI. We do not anymore connect to the digital earth pulsar, but instead create our own one. For this purpose, the image is now the apachepulsar/pulsar:2.6.2 image, and we use python 3.7.

For whatever reason, the venv module does not work in this container. So I just removed it, I think that's ok (as tox anyway creates it's own venv kind-of-thing).