Improve ATTRIBUTE_REMOVED sentinel to use class and more context

[potiuk]

ATTRIBUTE_REMOVED was a singleton object but then it made it difficult to find out which attribute has been removed. This PR changes the approach to allow for multiple AttributeRemoved objects as sentinel and compare class ratehr than object. The object contains name of the removed attribute. Better diagnostics at the expense of a bit more memory used.

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[potiuk]

>>> from airflow.utils.types import AttributeRemoved
>>> a = AttributeRemoved("aaaa")
>>> a.bbb
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/jarek/IdeaProjects/airflow/airflow/utils/types.py", line 60, in __getattr__
    raise RuntimeError(
RuntimeError: Attribute aaaa was removed on serialization and must be set again - found when accessing bbb.
>>> a.__class__ is AttributeRemoved
True
>>>

[potiuk]

cc: @dstandish