High priority: Add support for python 3.7

[fpgmaas]

Currently, only python 3.8, 3.9, and 3.10 are supported. Supporting Python 3.7 in the current setup does not work, due to this error:

So, because deptry depends on both flake8 (^4.0.1) and mkdocs (^1.3.0), version solving failed.

Probably mkdocs can be moved to a separate dependency group. This will require changes in the github actions workflow.

[fpgmaas]

This is now fixed by #27.

The first challenge was separating the environments for development and for publishing the documentation. This is now done, although the Github workflows can use some cleaning & refactoring. (added issue #28 for this).

This also required fixing of the unit tests, since the unit tests required numpy and pandas, which both only support Python >=3.8.