Remove license classifier

[gboutry]

While trying to upgrade the debian package, I'm prompted with the following error:

License classifiers have been superseded by license expressions (see https://peps.python.org/pep-0639/). Please remove:

License :: OSI Approved :: BSD License
make[1]: *** [debian/rules:21: override_dh_auto_install] Error 1

It seems license classifiers have been dropped in PEP-639, and have been superseeded by license and license-files properties, which are already present in the pyproject.toml