Remove SingleTaskGP type hint

Due to serialization issues:
python-attrs/cattrs#531

baybe/surrogates/gaussian_process.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, ClassVar, Optional
+from typing import TYPE_CHECKING, ClassVar
 
 from attr import define, field
 
@@ -12,7 +12,6 @@
 from baybe.surrogates.base import Surrogate
 
 if TYPE_CHECKING:
-    from botorch.models import SingleTaskGP
     from torch import Tensor
 
 
@@ -31,7 +30,9 @@ class GaussianProcessSurrogate(Surrogate):
     kernel: Kernel = field(factory=MaternKernel)
     """The kernel used by the Gaussian Process."""
 
-    _model: Optional[SingleTaskGP] = field(init=False, default=None)
+    # TODO: type should be Optional[botorch.models.SingleTaskGP] but is currently
+    #   omitted due to: https://github.com/python-attrs/cattrs/issues/531
+    _model = field(init=False, default=None)
     """The actual model."""
 
     def _posterior(self, candidates: Tensor) -> tuple[Tensor, Tensor]: