ENH: Import Bounds from scipy.optimize

Import `Bounds` from `scipy.optimize`.

Fixes:
```
src/nifreeze/model/gpr.py:32: error:
 Module "scipy.optimize._minimize" does not explicitly export attribute "Bounds"  [attr-defined]
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12437972140/job/34728973936#step:8:70

src/nifreeze/model/gpr.py

@@ -29,7 +29,7 @@
 
 import numpy as np
 from scipy import optimize
-from scipy.optimize._minimize import Bounds
+from scipy.optimize import Bounds
 from sklearn.gaussian_process import GaussianProcessRegressor
 from sklearn.gaussian_process.kernels import (
     Hyperparameter,