raise compiler error for 'cdef readonly class'

Cython/Compiler/Parsing.py

@@ -2026,9 +2026,11 @@ def p_c_class_definition(s, level, pos,
             error(pos, "Object struct name specification required for 'public' C class")
         if not typeobj_name:
             error(pos, "Type object name specification required for 'public' C class")
-    else:
+    elif visibility == 'private':
         if api:
             error(pos, "Only 'public' C class can be declared 'api'")
+    else:
+        error(pos, "Invalid class visibility '%s'" % visibility)
     return Nodes.CClassDefNode(pos,
         visibility = visibility,
         typedef_flag = typedef_flag,