Debugger doesn't trace into doctests

[pdawyndt]

The debugger doesn't run doctests, probably because it's instructed not to trace into modules (like testmod). Would however be highly relevant if student code were still traced while running the doctests. Of course we do not want to trace running the functionality of the `testmod' module itself.

[pdawyndt]

FYI: Here's a nice example of a solution where the original Python Tutor heavily suffers while rendering some frames with many objects/references, whereas the new implementation runs much more smoothly. As an additional advantage: the bug in this code can't be found in the Python Tutor due to the 1000-frame limit, where the new implementation is helpful because that limit is removed.

This sample code also has a doctest that could not be debugged as such: I had to inline the doctest as part of the code itself before I could start debugging.