py: now passes typechecks

This bases on Iggy's st3m/st4m unification branch, and is the first
large step towards actually unifying st3m/st4m.

python_payload.py now passes typechecks!

However, I had to rip out touch-based scroll controller code (for now
left in comments) as I didn't exactly understand how it works. But I'll
come back to it later.