The old sip based approach to handling property -> getter/setter
conversion is broken on sip6, so move the patching into Python.
Also fix incorrect checks in tests, and add additional test
for compatiblity code.
And make sipify handle this nicely. This means that all our non-flag
style enums correctly map across to IntFlag python enums on Qt 6,
fixing issues with negative enum values for these and providing
a better match for the original c++ enum.
