MacOS pipeline und manylinux wheels

Changes:

• Added MacOS build and test jobs
• build workflow is only run for the lowest supported python version
• test workflow only starts when build succeeds
• The wheel creation (right now only for linux) is a separate stage (because we only do one build in the build stage)

These changes will limit the number of running jobs quite a bit, if someone pushes changes that break the package, only 5 jobs (lint, docs, 3 * build) will start instead of 20.

Problems:

• The macOS pipeline does not work because tox somehow uses the system python instead of the brew installed one
• We might need a separate wheel directory for upload, or delete the old linux wheels, not sure how that will work out in the end
• The yaml has to be cleaned up a bit, but maybe that should be the last step