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The bug is in the copy_reg module.
object.__reduce_ex__ calls function 
_slotnames in that module.
There is another bug when __slots__
is a single string. Examples below.

>>> import copy_reg
>>> class A(object):
...     __slots__ = ('__spam',)
...
>>> class B(object):
...     __slots__ = 'spam'
...
>>> copy_reg._slotnames(A) # should be ['_A__spam']
['__spam']
>>> copy_reg._slotnames(B) # should be ['spam']
['s', 'p', 'a', 'm']