run-tests: use the same python version for shebang lines on Windows The latest py3 is used if the minor number isn't specified. After running the script to install all of the build dependencies, that moved the default from 3.8 to 3.9 on the CI system. That in turn caused a bunch of tests to be skipped that were running prior, even when the test runner was invoked with `py -3.8`. While we should almost always use the latest version, we really shouldn't make it hard to test different versions or allow things to randomly break in subtle ways like that. Differential Revision: https://phab.mercurial-scm.org/D10702