Made sdl2 text provider work (fixes for both python2 and python3)

Mostly understandable fixes, assuming I grok unicode right, ~~but I totally don't understand https://github.com/inclement/kivy/compare/kivy:master...inclement:sdl2-text?expand=1#diff-56ad8c59db4fc523c6b6470a45285458R12 - the relative import seems essential for it to work, otherwise it crashes, but I don't know why that would be.~~

Edit: python3 doesn't do implicit relative imports, which explains the original problem.