fix(typing): fix type annotations to match fix in marshmallow 3.24.0

Ref:
marshmallow-code/marshmallow@560eed7

Author: dairiki

marshmallow_dataclass/union_field.py

@@ -1,5 +1,6 @@
 import copy
 from typing import List, Tuple, Any, Optional
+from typing import Union as typing_Union
 
 import typeguard
 from typeguard import TypeCheckError
@@ -29,7 +30,9 @@ def __init__(self, union_fields: List[Tuple[type, fields.Field]], **kwargs):
         super().__init__(**kwargs)
         self.union_fields = union_fields
 
-    def _bind_to_schema(self, field_name: str, schema: Schema) -> None:
+    def _bind_to_schema(
+        self, field_name: str, schema: typing_Union[Schema, fields.Field]
+    ) -> None:
         super()._bind_to_schema(field_name, schema)
         new_union_fields = []
         for typ, field in self.union_fields: