make: default to using python3

This is what Mercurial does now.

--HG--
branch : stable
8 jobs for topic/stable/py3 in 5 minutes and 1 second (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #175553
checks-py2

00:01:21

passed #175554
checks-py3

00:01:44

passed #175559
doc

00:00:21

manual #175552
allowed to fail manual
pytype
passed #175555
tests-py2-cext

00:05:01

passed #175556
tests-py2-pure

00:03:54

passed #175557
tests-py3-cext

00:03:20

passed #175558
tests-py3-pure

00:04:12