Add debian package build for python-keycloak.

Associated git repository: https://forge.softwareheritage.org/source/python3-keycloak

Builds successfully tested locally.

I need a sysadmin intervention to setup the phabricator hook with that command.

$ ssh -p 2222 -t tate.internal.softwareheritage.org phabricator-setup-hook /srv/phabricator/repos/217/ post-receive-debian-deps

Related to T2302