refactor(utility): modify "AttrsMixin" as a workaround for pylint errors

`attr()` returns `Field` as the class variables value, using type hint
as the real type of variables.
pylint use `Field` as the variable type, so it will report the following
errors

`E0237: Assigning to attribute 'box2d' not defined in class slots
(assigning-non-slot)`
`E1101: Instance of 'Field' has no 'box3d' member (no-member)`

do the following as a workaround for pylint errors
* remove `__slot__` for E0237
* add `__getattr__` function for E1101

related issue:
pylint-dev/pylint#4341

PR Closed: Graviti-AI#580

tensorbay/utility/attr.py

@@ -33,7 +33,8 @@ class Field:  # pylint: disable=too-few-public-methods
 
     """
 
-    __slots__ = ("is_dynamic", "key", "default", "error_message", "loader", "dumper")
+    # remove __slots__ and add __getattr__ due to: https://github.com/PyCQA/pylint/issues/4341
+    # __slots__ = ("is_dynamic", "key", "default", "error_message", "loader", "dumper")
 
     def __init__(
         self, is_dynamic: bool, key: str, default: Any, error_message: Optional[str]
@@ -48,6 +49,11 @@ def __init__(
         if error_message:
             self.error_message = error_message
 
+    def __getattr__(self, name: str) -> None:
+        raise AttributeError(
+            _DEFAULT_ERROR_MESSAGE.format(class_name=self.__class__.__name__, attr_name=name)
+        )
+
 
 class AttrsMixin:
     """AttrsMixin provides a list of special methods based on attr fields.