Update to the latest upstream release

Please update the attrs package to the latest upstream release. The package has been also updated to use the new pybuild-plugin-pyproject.

Please note that changes involve all branches debian/master, upstream and pristine-tar (also tags have been updated).

All the changes are currently on https://salsa.debian.org/antonio.valentino/python-attrs but if the main uploaded agrees I could also push changes directly on the main salsa repository.