[setup] Update PyPI GPLv3 classifier designation

Fixes #254
4 jobs for 254_update_faulty_pypi_classifier_designation in 7 minutes and 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #191091
docker
build_linux_36

00:06:18

passed #191092
docker
build_linux_37

00:06:10

passed #191089
macosx
build_macosx_36

00:07:40

passed #191090
macosx
build_macosx_37

00:07:27