Fix hx71x on python2

[KevinOConnor]

This minor code change should fix the hx71x support on python2 installations. It also adds a load_cell regression test case.

@garethky , @alchemyEngine - fyi.

-Kevin

[Neko-vecter]

Python 2 support?? When will we fully migrate to python3?

-Neko.vecter

[hYdos]

We need to stop supporting python 2. It was EOL year's ago now and even suggesting klipper should have to support it is ridiculous