Fix typing of Union._serialize parameter

This tracks a change in type of the `attr` parameter to
`Field._serialize` made in marshmallow 3.18.0.

Here we also switch to using keyword args to typeguard.check_type
in an attempt to protect against upcoming
[changes in its signature](https://typeguard.readthedocs.io/en/latest/api.html#typeguard.check_type).

marshmallow_dataclass/union_field.py

@@ -34,13 +34,15 @@ def _bind_to_schema(self, field_name: str, schema: Schema) -> None:
 
         self.union_fields = new_union_fields
 
-    def _serialize(self, value: Any, attr: str, obj, **kwargs) -> Any:
+    def _serialize(self, value: Any, attr: Optional[str], obj, **kwargs) -> Any:
         errors = []
         if value is None:
             return value
         for typ, field in self.union_fields:
             try:
-                typeguard.check_type(attr, value, typ)
+                typeguard.check_type(
+                    value=value, expected_type=typ, argname=attr or "anonymous"
+                )
                 return field._serialize(value, attr, obj, **kwargs)
             except TypeError as e:
                 errors.append(e)
@@ -53,7 +55,9 @@ def _deserialize(self, value: Any, attr: Optional[str], data, **kwargs) -> Any:
         for typ, field in self.union_fields:
             try:
                 result = field.deserialize(value, **kwargs)
-                typeguard.check_type(attr or "anonymous", result, typ)
+                typeguard.check_type(
+                    value=result, expected_type=typ, argname=attr or "anonymous"
+                )
                 return result
             except (TypeError, ValidationError) as e:
                 errors.append(e)