2.2: Support for RGB and other fixes

Update README.md with note about cython3 (#45, @Decker108)
Set global UTF-8 locale (#47, @matemaciek)
Support for setting colour in Kelvin for the whole spectrum (#48, @matemaciek)
Fix git URL in aiocoap.sh (#49, @cosmix)
Add sRGB to xyY conversion (#51, @r41d)