ENH: Avoid type checking for private function import statement

Avoid type checking for private function import statement.

Fixes:
```
src/nifreeze/model/gpr.py:215: error:
 Module "sklearn.utils.optimize" has no attribute "_check_optimize_result"  [attr-defined]
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12437972140/job/34728973936#step:8:73

src/nifreeze/model/gpr.py

@@ -212,7 +212,7 @@ def _constrained_optimization(
     ) -> tuple[float, float]:
         options = {}
         if self.optimizer == "fmin_l_bfgs_b":
-            from sklearn.utils.optimize import _check_optimize_result
+            from sklearn.utils.optimize import _check_optimize_result  # type: ignore
 
             for name in LBFGS_CONFIGURABLE_OPTIONS:
                 if (value := getattr(self, name, None)) is not None: