Numpy deprecation warning issued from within `test_metropolis.py`

### Description

```python
 tests/step_methods/test_metropolis.py: 15950 warnings
  D:\a\pymc\pymc\pymc\backends\ndarray.py:118: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)
    data[key][self.draw_idx] = val
```