Switch from python3-gpgme to python3-gpg

python3-gpgme is gone from testing/unstable, python3-gpg is the better alternative, it's provided as part of gpgme upstream. See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=866024

We should update distro-tracker to rely on python3-gpg and drop python3-gpgme. I can't find any official documentation of new Python API but it will likely be close to the one of the C library: https://www.gnupg.org/documentation/manuals/gpgme/