Sleeve Case for Raspberry Pi: Heatsink Edition

After using the excellent [sleeve design](https://www.thingiverse.com/thing:604915) from [walter](https://www.thingiverse.com/walter), I noticed adding heatsinks to the Pi (at least Pi 3 B+) would prevent the sleeve from fitting due to clearance issues.

This remix simply adds space for the cpu heatsink, specifically the dimensions of ones provided by popular Pi kits by CanaKit. I didn't want to raise the height of the entire sleeve as I think that would diminish much of its aesthetic appeal.