Aptitude provided the following error message without this fix:
 mariadb-client-core-10.3 :
    Conflicts: mysql-client-5.1 which is a virtual package, provided by:
             - mariadb-client-10.3, but 10.3.0+maria~xenial is to be installed.
    Conflicts: mysql-client-5.5 which is a virtual package, provided by:
             - mariadb-client-10.3, but 10.3.0+maria~xenial is to be installed.