Handle foreign data in U. F. O..

This uses the PyTuple, which has no matching entity in X. M. L., as a special container for unrecognized data in python_persistent, allowing FontForge to round-trip that data in the top-level lib.plist and in the lib data for individual glifs.

@adrientetar, I expect there to be a conditional on that block at some point. There are inevitably cases in which we will want to abort parsing some block, I just don't know what they are.