Allow python3 system python #26953
Description
On a python3-only system its likely that there is no python command, since PEP394 argues for /usr/bin/python to be python2 only for now. In particular, the Debian 10 buildbot doesn't have python (since it uses python3 buildbot).
The problem is that our sage-system-python script only looks for python, it should fall back to python3
CC: @embray @fchapoton
Component: build
Author: Frédéric Chapoton
Branch: b7c84b7
Reviewer: Volker Braun
Issue created by migration from https://trac.sagemath.org/ticket/26953