fix: handle deprecation in pydantic 2.10

Otherwise we get:

```
pydantic.warnings.PydanticDeprecatedSince211: Accessing the 'model_fields' attribute on the instance is deprecated. Instead, you should access this attribute from the model class. Deprecated in Pydantic V2.11 to be removed in V3.0.
```

Author: cjwatson

src/pydantic_compat/_shared.py

@@ -8,6 +8,7 @@
 from packaging.version import Version
 
 PYDANTIC2 = Version(pydantic.version.VERSION) >= Version("2")
+PYDANTIC2_10 = Version(pydantic.version.VERSION) >= Version("2.10")
 FIELD_KWARGS = {
     p.name
     for p in signature(pydantic.Field).parameters.values()

src/pydantic_compat/_v2/mixin.py

@@ -5,7 +5,11 @@
 from pydantic import BaseModel
 from pydantic._internal import _model_construction
 
-from pydantic_compat._shared import V2_RENAMED_CONFIG_KEYS, check_mixin_order
+from pydantic_compat._shared import (
+    PYDANTIC2_10,
+    V2_RENAMED_CONFIG_KEYS,
+    check_mixin_order,
+)
 
 if TYPE_CHECKING:
     from pydantic import ConfigDict
@@ -93,7 +97,10 @@ def parse_raw(cls: type[Model], *args: Any, **kwargs: Any) -> Any:
     # this is needed in addition to the metaclass patch in __init_subclass__
     @property
     def __fields__(self: Model) -> Dict[str, Any]:  # noqa: UP006
-        return self.model_fields
+        if PYDANTIC2_10:
+            return self.__class__.model_fields
+        else:
+            return self.model_fields
 
     @property
     def __fields_set__(self: Model) -> set[str]:

tests/test_base_model.py

@@ -4,6 +4,7 @@
 import pytest
 
 from pydantic_compat import PYDANTIC2, PydanticCompatMixin
+from pydantic_compat._shared import PYDANTIC2_10
 
 
 class Model(PydanticCompatMixin, pydantic.BaseModel):
@@ -64,7 +65,8 @@ def test_v1_attributes() -> None:
 
 def test_v2_attributes() -> None:
     m = Model()
-    assert "x" in m.model_fields
+    if not PYDANTIC2_10:
+        assert "x" in m.model_fields
     assert "x" in Model.model_fields
     assert "x" not in m.model_fields_set
     m.x = 2