Fix AuthClient.create_policy signature

[sirosen]

I've taken another iteration on this approach to strip it all the way down to the bare essentials.
The new definition puts all of the compatibility code into the decorator. That means

• the definition is now entirely correct in v3, and in v4 we can just remove the compat decorator
• this approach to shimming lets us do a very simple zip rather than inspection of locals

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This is done with a compatibility wrapper which unpacks arguments to ensure that legacy (positional-argument usage) is supported under SDK v3.

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📚 Documentation preview 📚: https://globus-sdk-python--936.org.readthedocs.build/en/936/