Submit sip v6 fix upstream

[robwoolley]

I produced a fix for sip to report the correct Python suffix when building: https://github.com/robwoolley/meta-openembedded/commit/3c4c1e8cd25265fcad31346d007f71f8cc8c7a45

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: https://github.com/ros/meta-ros/commit/3c007dc7e85dc378ae12e2e500cb998972e047f9

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