STYLE: Remove ClassVar[dict] type hinting from GPR param constraints

Remove `ClassVar[dict]` type hinting from GPR parameter constraints
variable and define it as a normal dictionary.

Fixes:
```
Cannot override instance variable '_parameter_constraints'
 (previously declared on base class 'GaussianProcessRegressor') with class variable
```

As `_param_constraints` is an instance variable in `scikit-learn`:
https://github.com/scikit-learn/scikit-learn/blob/1.6.X/sklearn/gaussian_process/_gpr.py#L189

Author: jhlegarreta

src/nifreeze/model/gpr.py

@@ -25,7 +25,7 @@
 from __future__ import annotations
 
 from numbers import Integral, Real
-from typing import Callable, ClassVar, Literal, Mapping, Optional, Sequence, Union
+from typing import Callable, Literal, Mapping, Optional, Sequence, Union
 
 import numpy as np
 import numpy.typing as npt
@@ -156,7 +156,7 @@ class DiffusionGPR(GaussianProcessRegressor):
 
     optimizer: Optional[Union[StrOptions, Callable, None]] = None
 
-    _parameter_constraints: ClassVar[dict] = {
+    _parameter_constraints = {
         "kernel": [None, Kernel],
         "alpha": [Interval(Real, 0, None, closed="left"), np.ndarray],
         "optimizer": [StrOptions(SUPPORTED_OPTIMIZERS), callable, None],