DVM.HasValue is now virtual, and controls whether GetValue will be called.

[RobinD42]

Operating system: any
wxPython version & source: 4.1+
Python version & source: any

Description of the problem:

See #1594
Depends on wxWidgets/wxWidgets#1792

Update wxWidgets reference to include the DVM.HasValue changes, and include some minor tweaks to support that in wxPython.

[RobinD42]

This issue has been mentioned on Discuss wxPython. There might be relevant details there:

https://discuss.wxpython.org/t/wxpython-4-1-1-released/35043/1