fix: complete Python 3.8 compatibility fixes

- Fix tuple[Type[Any], ...] to Tuple[Type[Any], ...] in shared.py
- Add missing _is_model_field function to v2.py
- All tests now pass on Python 3.8 without TypeError
- Import-time compatibility fully restored for Python 3.8

fastapi/_compat/shared.py

@@ -97,7 +97,7 @@ def value_is_sequence(value: Any) -> bool:
 
 
 def _annotation_is_complex(annotation: Union[Type[Any], None]) -> bool:
-    types_tuple: tuple[Type[Any], ...] = (BaseModel, Mapping, UploadFile)
+    types_tuple: Tuple[Type[Any], ...] = (BaseModel, Mapping, UploadFile)
     if "pydantic.v1" in sys.modules:
         # only now touch v1 (already used by app)
         types_tuple += (_v1.BaseModel,)

fastapi/_compat/v2.py

@@ -471,3 +471,8 @@ def get_long_model_name(model: TypeModelOrEnum) -> str:
 def _is_model_class(value: Any) -> bool:
     """Check if a value is a Pydantic model class."""
     return lenient_issubclass(value, BaseModel)
+
+
+def _is_model_field(value: Any) -> bool:
+    """Check if a value is a Pydantic model field."""
+    return isinstance(value, ModelField)