Fix for .NET 9

Author: slozier

Src/IronPython/Runtime/Types/NewTypeMaker.cs

@@ -531,6 +531,7 @@ private void ImplementCustomTypeDescriptor() {
             ImplementInterface(typeof(ICustomTypeDescriptor));
 
             foreach (MethodInfo m in typeof(ICustomTypeDescriptor).GetMethods()) {
+                if (!m.IsAbstract) continue;
                 ImplementCTDOverride(m);
             }
         }