The last time I read up on it, the ZW thing has been around since the beginning of time as far as we can tell. Some point in evolution Mammals developed the XY in a common ancestor to both snake and humans. I bring this up so some can understand this happen a really long time ago and it's not something that just happens all the time. Switching to XY would be ground breaking, it's not just a random monogenetic mutation like pastel, it would be something that disrupts a pattern hundreds of millions of years long. Given we have other systems, it obviously is not impossible. But to think ball pythons managed to make the switch has to be taken with a pile of salt. I think most would need a little more proof than a gene that only fits with the other model with a pretty high crossing over rate on top of it.