[3.7] bpo-34621: fix uuid.UUID (un)pickling compatbility with older Python versions (<3.7)

[taleinat]

This is a backport of part of the implementation of bpo-30977 in PR #9106, which will not be backported in its entirety.

https://bugs.python.org/issue34621

[serhiy-storchaka]

In Python 3.6 and older the UUID object can have additional attributes, which will be preserved with pickling. I don't know whether anybody uses this, and don't think we should add tests for this, but it is better to avoid breaking in maintained release without need. I would write the following code:

    def __getstate__(self):
        state = self.__dict__
        if self.is_safe != SafeUUID.unknown:
            # is_safe is a SafeUUID instance.  Return just its value, so that
            # it can be un-pickled in older Python versions without SafeUUID.
            state = {**state, 'is_safe': self.is_safe.value}
        return state

    def __setstate__(self, state):
        self.__dict__.update(state)
        # is_safe was added in 3.7; it is also omitted when it is "unknown"
        self.__dict__['is_safe'] = (SafeUUID(state['is_safe'])
                                    if 'is_safe' in state else SafeUUID.unknown)

[taleinat]

Great catch! I've addressed this in the latest commit.

Note: I went with a more verbose spelling for overriding is_safe in __getstate__ which is IMO easier to read.