plugin_registry: fix always-true misuse of assert stmt
    
    Apparently, comma-separated values -> tuple is preceded with assert
    statement syntactic closure recognition, hence pair of arguments
    to assert cannot be enclosed with parentheses (previous assumption was
    that it is the same due to syntactically both notations are equivalent).
    
    Reported-by: Tomáš Jelínek" <tojelinek@redhat.com>
    Signed-off-by: Jan Pokorný <jpokorny@redhat.com>