move to python3 to build ffi_build.py
move to python3 to build ffi_build.py

I don't really use python2 anything anymore, and xenial doesn't even have
it by default. Let's use python3 for this.

Signed-off-by: Tycho Andersen <6b523322678240882acbb4da5312a12d8cd0082d@tycho.ws>

tych0/xcffib