Extend / Fix the "NDB_PY2_UNPICKLE_COMPAT" flag

[kaansoral]

'latin1' encoding is the fix to most common fix of the DecodeError's, throughout the docs it's the adopted fix as well, which can be seen here: https://cloud.google.com/appengine/docs/standard/python3/services/access

After this fix, I believe the "NDB_PY2_UNPICKLE_COMPAT" flag should be added to the official docs as well

Fixes #111

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[pnico]

I filed issue #103 asking basically "what is this thing" and got no responses so good luck ever getting this merged :/

If you don't want to use a modified appengine lib, you could try something like this (YMMV, no guarantees)

try:
    _ = self.data
except UnicodeDecodeError:  # we know there's data, probably stored in Python 2
    try:
        prop = self._properties.get('data')
        self.data = pickle.loads(prop._get_base_value(self).b_val, encoding='latin1')
        # we 'fixed' it, we can use self.data now
    except Exception:
        # couldn't fix it, could raise here or just log something scary and give up:
        self.data = None

[kaansoral]

Thank you, this was the response to my second issue tracker issue: https://issuetracker.google.com/issues/322838721

I do share the same sentiment with you, I doubt this will be fixed for others too, I suspect we all push the wagon ourselves at this point those who are left

Have no idea what 'bytes' encoding is for, but I kept it in place so that whatever edge case it fixed, is still fixed