Improve continuous integration workflow

[gnn]

With these changes

• the CI badge is now a link to the GitHub workflow details,
• the CI badge's alt text is no longer specific to GitHub,
• the list of cleaned files is now a module global variable in "noxfile.py",
• the CI checks are done in multiple small steps instead of one monolithic one,
• packaging is checked on multiple Python versions,
• package installation is now checked, too (and also on multiple versions),
• the workflow job names are now shorter,
• the workflow jobs are ordered alphabetically in GitHub's UI (this means Linux comes first, yay ;) ), but most importantly
• the CI checks pass again, because the cleaned files have been reformatted with the most recent version of black.