netcmd/user: python[3]-gpgme unsupported and replaced by python[3]-gpg

netcmd/user: python[3]-gpgme unsupported and replaced by python[3]-gpg

python[3]-gpgme is deprecated since ubuntu 1804 and debian 9.
use python[3]-gpg instead, and adapt the API.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13728

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit 84069c8a5476a47d45ab946d82abb0d6c04635c3)