Requirements splitting and python version

The same issue as it has been with VRE Language package:

See merge requests https://git.scc.kit.edu/virtmat-tools/vre-language/-/merge_requests/46 and https://git.scc.kit.edu/virtmat-tools/vre-language/-/merge_requests/47

In addition, is there a reason to require python 3.7? The current system-default version on HoreKa and bwUniCluster is 3.6.8.

https://git.scc.kit.edu/virtmat-tools/vre-middleware/-/blob/master/setup.cfg#L13