Python3 update 1 2 3

Pretty much the same as these updates on the `python` package : https://github.com/openwrt/packages/pull/438  https://github.com/openwrt/packages/pull/445 and  https://github.com/openwrt/packages/pull/449

Quick highlights:
- enforce existence of files in filespecs
- switch to python exec + python shared lib
- disable Python tests during build

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>