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IntEnum is advertised as being a drop-in replacement for integer contants; however this fails in the case of unpickling on previous Python versions.
This occurs because when a pickle is created the module, class, and value are stored -- but those don't exist prior to Python 3.4.
One solution is to modify IntEnum to pickle just like a plain int would, but this has the serious disadvantage of losing the benefits of IntEnum on Python versions that support it.
Another solution is to use Serhiy's idea of pickling by name; this would store the module and name to look up in that madule, and this works on all Python versions that have that constant: on Python 3.3 socket.AF_INET returns an integerer (2, I think), and on Python 3.4+ it returns the AF_INET AddressFamily member.

Patch adds Enum._convert which is a class method that handles:
 - creating the new Enum
 - adding the appropriate members
 - adding the new Enum to the module's namespace (which is a passed parameter)
 - replacing the __reduce_ex__ method to return just the member's name
The change to Enum is:
 @classmethod
 def _convert(cls, name, module, filter, source=None):
 """
 Create a new Enum subclass that replaces a collection of global constants
 """
 # convert all constants from source (or module) that pass filter() to
 # a new Enum called name, and export the enum and its members back to
 # module;
 # also, replace the __reduce_ex__ method so unpickling works in
 # previous Python versions
 module_globals = vars(sys.modules[module])
 if source:
 source = vars(source)
 else:
 source = module_globals
 members = {name: value for name, value in source.items()
 if filter(name)}
 cls = cls(name, members, module=module)
 cls.__reduce_ex__ = _reduce_ex_by_name
 module_globals.update(cls.__members__)
 module_globals[name] = cls
 return cls
In use it looks like:
 IntEnum._convert(
 'AddressFamily',
 __name__,
 lambda C: C.isupper() and C.startswith('AF_'))
or
 _IntEnum._convert(
 '_SSLMethod', __name__,
 lambda name: name.startswith('PROTOCOL_'),
 source=_ssl)
ssl.py, socket.py, signal.py, and http/__init__.py have been updated to use this method.

New changeset c93d46fd85ee by Ethan Furman in branch 'default':
issue23673
https://hg.python.org/cpython/rev/c93d46fd85ee 

New changeset 5dd7b5fb65e7 by Ethan Furman in branch '3.4':
issue23673
https://hg.python.org/cpython/rev/5dd7b5fb65e7