use black and ruff

• as discussed last week with Ashwin and Martin
• line length is 88 (i.e. black default and compatible with ruff) by default
• black changes and ruff checks are complementary (so we need both unless we change the settings).
• I created a Makefile with central targets to make the use of these tools easier for occasional contributors (so they do not need to remember the detailed "py.test" syntax), and call these targets from the .gitlab-ci.yml file. On balance, I think that is useful. The Makefile has short comments to explain what each target does.
• some of the white space changes are from black - they distract from the changes on .gitlab-ci.yml and pyproject.toml but are needed to make the CI pass.