Run flake8 and unittests on pullrequests and pushes

Created by: matthijskooijman

This sets up Github Actions to automatically run flake8 and unittests on pullrequests and direct pushes. Test results should show up below the pullrequest and the author might get an e-mail notification (at least for direct pushes to this repository, not sure about pullrequests).

There are some additional commits to simplify running the test suite and document this in the README.

This PR assumes that the minimum supported Python version is 3.5, so it runs tests and flake8 under 3.5. Since this assumption is not currently true, these tests will now fail until #601 is merged. The testsuite is also ran under a newer Python version, which should already succeed now.

Since the code is not currently pep8-compliant, flake8 is run with a lot of ignores set, to allow it to complete succesfully (after #601 is merged). This is not ideal, but until we fix these pep8 issues, this check will at least ensure that no new kinds of violations will be introduced.