This line in copyreg._slotnames does not properly calculate mangled attribute names:
https://github.com/python/cpython/blob/v3.6.2/Lib/copyreg.py#L131

The problem is that it does not strip leading underscores from the class name:

>>> class _LeadingUnderscoreClassName(object):
...     __slots__ = ("__bar",)
...
>>> import copy_reg
>>> copy_reg._slotnames(_LeadingUnderscoreClassName)
['__LeadingUnderscoreClassName__bar']

The result is that copy, pickle, and anything else that relies on _slotnames() do not work on classes with leading underscores and private __slots__. This bug is present in all versions of Python.