After some hard debugging:
- doctest.debug_src() is unlucky enough to trigger a garbage collection
just when compiling the given code.
- gc collects unreachable objects, among them is an instance of the
class doctest._SpoofOut, which derives from io.StringIO.
- The debugger steps into io.IOBase.__del__

Some possible directions:
- Change the gc thresholds. A very temporary workaround to make the test
pass.
- Find the cycle involving the SpoofOut object, and try to break it in
doctest.py.
- Find a way to disable pdb tracing when the gc is running finalizers.
(this is what happens in 2.5: pdb does not step into a C function)
- Forget everything, and wait for the io.py object to be rewritten in C.