An update: Using Miniterm running on the Pi, I've been able to watch data arrive in perfect condition from the xBee connected thru the GPIO, thus confirming that the two devices are connected correctly. However, the issue of reading the data in a Python, or any other kind of script, remains unsolved.
As I've not seen a working solution to this problem anywhere (either maker shops, tech spaces, college research labs, or through Web searches), the question becomes whether I should abandon the GPIO approach and connect the xBee through the USB port, perhaps using the Adafruit FTDI friend (
https://www.adafruit.com/products/284) instead?
Thanks again to pburgess and the admins for your excellent response and guidance on this challenge.