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This version addresses compatibility with the newer versions of the PayPal API
by backing off on pre-query validation. PayPal's API error messages are
the safest bet, as bad as they can be at times.
Lots of cleanup, commenting, and creature comforts abound. Backwards-incompatible
changes are prefixed with 'BIC'.
* BIC: Removed the KEY_ERROR setting. Less complexity is better. (gtaylor)
* BIC: The 'token' position argument to do_express_checkout_payment
is no a keyword arg, for the sake of consistency. (gtaylor)
* Adding lots of logging, which users may attach to for debugging. (gtaylor)
* Removed RESPONSE_KEYERROR. Wasn't being used. (gtaylor)
* Removed DEBUG_LEVEL setting. Use Python logging. (gtaylor)
* Removed some of the error checking on input values, as this
limits users to the old API versions. We'll have to rely
on PayPal's error messages being informative. (gtaylor)
* Bumped the default API version protocol to 72.0, up from 53.0. (gtaylor)