Stop tracking function calls that returned a value

[pdawyndt]

Steps to reproduce:

• go to exercise [])()
• submit the sample solution
• launch the debugger from any test case
• you'll get

There are many calls to the function isISBN that returned a value, but are still tracked by the Python Tutor. This is actually an issue that is inherited from the original Python Tutor. But it results in a state explosion of the debug frames, which is why debugging takes much more time than needed.

You'll have to reduce the length of the list when running the test code in the original Pytbon Tutor, as it has a hard limit of 5600 bytes for the length of code samples.