Upgrade to Python3.8

[zbynekwinkler]

The sooner we upgrade, the longer we have to fix possible regressions. I believe Python3.8 is available on all platforms we currently use (sudo apt install python3.8 for ubuntu-18.04) and it is going to be the default python3 on ubuntu-20.04.

The blocker here is the availability of a working wheel realsenseai/librealsense#6126.