missing @DataClass decorator format fix

Author: p1c2u

Diff for: openapi_core/unmarshalling/schemas/exceptions.py

@@ -20,6 +20,7 @@ class FormatterNotFoundError(UnmarshallerError):
     def __str__(self) -> str:
         return f"Formatter not found for {self.type_format} format"
 
+
 @dataclass
 class FormatUnmarshalError(UnmarshallerError):
     """Unable to unmarshal value for format"""