Port to Python3

Python 2 is EOL and it is increasingly hard to justify keeping Py2-dependent packages in distributions. A comment in the project's CMake indicates that potential porting work was stalled on how to 'differentiate' between 2 and 3, I assume that is about providing libs for both: https://invent.kde.org/libraries/kross-interpreters/-/blob/master/CMakeLists.txt#L48

As far as I am aware, the only consumer of krosspython.so these days is lokalize. Maybe it would be best to make a clean cut and port both lokalize scripts and kross-interpreters to Py3?

The first step would be to upgrade the bundled PyCXX to latest release 7.1.4 for py38/39 support and add the necessary Python3 dirs that are currently missing.