Revise handling of package metadata and build-system

This MR addresses some housekeeping on the package-level:

1. Switch from a pure setup.py build to a PEP517/518 compatible system using setup.cfg and pyproject.toml
2. Set package-wide defaults for flake8 and pytest
3. Add a gitignore file

.gitlab-ci.yml

@@ -4,7 +4,6 @@ pytest:
   stage: test
   before_script:
     - python3 -m pip install -r requirements.txt
-    - python3 -m pip install pytest
   script:
     - pytest --doctest-modules --junitxml=report.xml
   artifacts: