CI/CD: black and flake8 are not running in our pipeline

The three script: tags in .gitlab-ci.yml are overwriting one another so that only the last one (unit tests) is being run. Black and flake8 are properly installed in our CI process but they are never being run.

This can be seen by pasting the contents of .gitlab-ci.yml into https://gitlab.com/<your GitLab username>/pymarc/-/ci/lint and clicking validate. The .yml file is valid but unfortunately the black and flake8 job steps are absent from the validated output.