Enable black for xplat/yoga
Enable black for xplat/yoga

Summary:
This directory was excluded from Black formatting in D14888843 when Black
was turned on for all of `xplat/`. Since there aren't a lot of `.py` files in
this directory, the opt-out is being removed, and the outstanding issues fixed.

Reviewed By: d16r

Differential Revision: D14889104

fbshipit-source-id: 440077c1efcb4c653151bca1da5636212978add5

facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/yoga,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/css-layout,facebook/css-layout