contrib: install latest versions of Python
Let's keep our build environment modern.
We upgrade 3.5-3.8 on Linux. Just 3.7 and 3.8 on Windows because we
don't use <3.7 on Windows.
Differential Revision: https://phab.mercurial-scm.org/D9145
@echo off
powershell -NoProfile -ExecutionPolicy unrestricted -Command "& '%~dp0\editmergeps.ps1' %*"