The issue is that the wx.ListCtrl class has gained a CheckItem method which has a slightly different API. So when you call CheckItem it’s finding the one in wx.ListCtrl first in the MRU instead of the one in the mixin class.
https://docs.wxpython.org/wx.ListCtrl.html#wx.ListCtrl.CheckItem
I thought I had implemented a fix or workaround for this, but I can’t seem to find it now. Perhaps it was on a branch that didn’t get merged. Anyway, since the wx.ListCtrl is now able to provide checkboxes then one option is to drop using the mixin class, and switch to using the built-in checkbox support instead. You just need to call EnableCheckBoxes first.