Eventually update build.py to remove xAvgCharWidth workaround once fonttools is fixed

Once fonttools fixes fonttools/fonttools#2538 then

Line 152 in 180fac5

    
           # Irritatingly, pyftmerge does not calculate the average character width correctly. It appears to simply average the xAvgCharWidth value in the two font files. However, given the heavy weighting of wider Japanese characters, we need to recalculate the average and insert it back into the font.

can be updated

@aaronbell I suggested Simon could fix this in that FT issue... who else do you think would be good to get this sorted out?

@davelab6 if you’re OK with just lifting the code that I used (which essentially follows the FB check code), I can probably submit a fix for this?

I imagine that everyone has seen this by now but just in case, FontTools released a fix: https://github.com/fonttools/fonttools/releases/tag/4.33.0