ENH: Use ClassVar for class variable type hinting

Use `ClassVar` for type hinting the `GaussianProcessRegressor`
`_parameter_constraints` class variable in child class.

Fixes:
```
src/nifreeze/model/gpr.py:156: error:
 Cannot override class variable (previously declared on base class "GaussianProcessRegressor") with instance variable  [misc]
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12437972140/job/34728973936#step:8:71

src/nifreeze/model/gpr.py

@@ -25,7 +25,7 @@
 from __future__ import annotations
 
 from numbers import Integral, Real
-from typing import Callable, Mapping, Sequence
+from typing import Callable, ClassVar, Mapping, Sequence
 
 import numpy as np
 from scipy import optimize
@@ -153,7 +153,7 @@ class DiffusionGPR(GaussianProcessRegressor):
 
     """
 
-    _parameter_constraints: dict = {
+    _parameter_constraints: ClassVar[dict] = {
         "kernel": [None, Kernel],
         "alpha": [Interval(Real, 0, None, closed="left"), np.ndarray],
         "optimizer": [StrOptions(SUPPORTED_OPTIMIZERS), callable, None],