Doing import sys;print(sys.executable) returned Python 2.7
I had selected Python27 interpreter on my project's properties, but when I removed the Python31 interpreter from the main list the debugger wouldn't run, complaining that it couldn't find Python31. So I went through every configuration screen I cound think of, and re-selected Python27 everywhere (some of them had "Default Interpreter" selected). It finally got going.