Fix: assertion was too strict.

Author: fangerer

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/classes/IsSubtypeNode.java

@@ -94,8 +94,8 @@ protected boolean isSubMro(Object base, MroSequenceStorage derivedMro, int baseM
         int offset = derivedMroLen - baseMroLen;
         if (offset >= 0) {
             // we can only do this for classes where all MRO entries have only a
-            // single base
-            assert GetBaseClassesNodeGen.getUncached().execute(derivedMroAry[offset]).length == 1;
+            // single or no base
+            assert GetBaseClassesNodeGen.getUncached().execute(derivedMroAry[offset]).length <= 1;
             return isSameType(isSameTypeNode, derivedMroAry[offset], base);
         } else {
             return false;