Pure Python Library for Geometry Operations

I don't know about GAE compatibility, but you migh try SymPy for pure math & geometry functions. Otherwise, the only other one I know about is Shapely but it has a libgeos_c dependency which might disqualify it based on your requirements.

UPDATE: Also SymPy is BSD licensed, which might be an important benefit to some devs.

Planar has very limited support for topology operations (only convex hull and point-in-polygon) but it is pure Python and for some cases it could be useful.