Submit sip v6 fix upstream

[robwoolley]

I produced a fix for sip to report the correct Python suffix when building:
robwoolley/meta-openembedded@3c4c1e8

However, before submitting it upstream I found that I could not reproduce the problem. I think this additional fix to qt-gui-cpp where I manually set PYTHON_EXTENSION_MODULE_SUFFIX may be hiding the fault: 3c007dc

Need to take a closer look whether the sip v6 fix still needs to be submitted upstream.