This is the code I'm using :
from kivy.core.text import LabelBase
LabelBase.register('Roboto',
'fonts/Roboto.ttf',
'fonts/RobotoOblique.ttf',
'fonts/RobotoBold.ttf',
'fonts/FreeSansBoldOblique.ttf')
Would allow your code above to work if the above listed fonts are in the paths specified i.e. a "fonts" subfolder.