wxPython is now on version 4.1.1, so I’d say at least 4.0.
Bringing up the py3 branch to have a look at where the UI is, one of the first things I tried was File -> New. This immediately died. It seems that the recently added code for maintaining a recently-opened list relies on a function that was present in old versions of wxPython (e.g. 2.8.x) but has been supplanted by a more flexible set of functions in the latest version (i.e. Phoenix).
This raises the question of what the target version of wxPython is intended to be. There is a comment in the recently added developer module related to Phoenix, so I'd assume that it's intended to work under Phoenix. The question is whether that's what we should be working with, or if support for an older version is also required.
The particular issue I encountered is easily fixed for 2.8.6.0 and later, but I think it would be worth putting a stake in the ground in specifying a minimum version of wxPython as a target, so that people know what they should develop with.