[mlir] Use the correct base class for Attributes in Python bindings

[ftynse]

Summary

The pybind class typedef for concrete attribute classes was erroneously
deriving all of them from PyAttribute instead of the provided base class. This
has not been triggering any error because only one level of the hierarchy is
currently exposed.

Depends On D89778

[stellaraccident]

Thanks - missed this in the previous fix to the type hierarchy.

mlir/lib/Bindings/Python/IRModules.cpp

 /// concrete attribute class extends PyAttribute); however, intermediate
 /// python-visible base classes can be modeled by specifying a BaseTy.
 template <typename DerivedTy, typename BaseTy = PyAttribute>
 class PyConcreteAttribute : public BaseTy {
 public:
   // Derived classes must define statics for:
   //   IsAFunctionTy isaFunction
   //   const char *pyClassName
-  using ClassTy = py::class_<DerivedTy, PyAttribute>;
+  using ClassTy = py::class_<DerivedTy, BaseTy>;
   using IsAFunctionTy = int (*)(MlirAttribute);
 
   PyConcreteAttribute() = default;
   PyConcreteAttribute(PyMlirContextRef contextRef, MlirAttribute attr)
       : BaseTy(std::move(contextRef), attr) {}
   PyConcreteAttribute(PyAttribute &orig)
       : PyConcreteAttribute(orig.getContext(), castFrom(orig)) {}