Fix 410807: Python Scripter Debugger won't run (Linux)

This fixes the issues I got on Linux. Before I try to tackle Windows, I'm going to wait for the nightly build and see if the behaviour on Windows is still the same with this change.

The 'yield from' syntax has been introduced in Python 3.3, so this won't run on older Python versions, including 2.x. According to https://vfxplatform.com/ we can drop Python 2 support in 2020, and Boud said to drop Python 2 support now in master, seeing how Krita 4.3 will come out the end of 2019 anyway.