Can not iterate on values attribute of a Variable with PyObject dtype.

[YooSunYoung]

This works fine as expected:

var = sc.concat([sc.scalar([1,2,3])]*3, dim='x')
var.values
#  [[1, 2, 3], [1, 2, 3], [1, 2, 3]]

But this one fails:

var = sc.concat([sc.scalar([1,2,3])]*3, dim='x')

for val in var.values:
    print(val)

with this error messages:

TypeError: Unregistered type : scipp::python::PyObject

The above exception was the direct cause of the following exception:

TypeError                                 Traceback (most recent call last)
Cell In[22], [line 3](vscode-notebook-cell:?execution_count=22&line=3)
      [1](vscode-notebook-cell:?execution_count=22&line=1) var = sc.concat([sc.scalar([1,2,3])]*3, dim='x')
----> [3](vscode-notebook-cell:?execution_count=22&line=3) for val in var.values:
      [4](vscode-notebook-cell:?execution_count=22&line=4)     print(val)

TypeError: Unable to convert function return value to a Python type! The signature was
	(self: pybind11_builtins.iterator) -> scipp::python::PyObject

I came across this error because the Mtz file (for NMX) has a data column of lists(miller indices),
and I needed to convert them into string as a workaround for grouping...