PerformanceMonitoring: First batch of cmake, flake8, clean-up updates

This relates to ATEAM-678. The original goal was to enable flake8 for the packages under Control/PerformanceMonitoring. While doing so, I tried to clean things up that I'm fairly certain not used and/or needed anymore, although I played it safe. I also went through the cmake configurations, fixed a number of py3 problems in some scripts that are not used very frequently etc. It turned out to be a bit more massive MR than I was intending but pretty much all changes are either stylistic and/or uncontroversial. There is essentially no new functionality.

BTW, there are a few commits but it would be nice if we don't squash them just in case.