Port fix for .NET 9 from IronLanguages/ironpython3#1807 to ironpython2

Author: DawidPietrykowski

Src/IronPython/Runtime/Types/NewTypeMaker.cs

@@ -540,6 +540,7 @@ private void ImplementCustomTypeDescriptor() {
             ImplementInterface(typeof(ICustomTypeDescriptor));
 
             foreach (MethodInfo m in typeof(ICustomTypeDescriptor).GetMethods()) {
+                if (!m.IsAbstract) continue;
                 ImplementCTDOverride(m);
             }
         }