autotools: allow python3 and prefer it if both are available
In case --with-python is passed we allow 2.7+ and 3.5+. In case nothing is passed we try to find 3.5+ first and then fall back to 2.7+
In case --with-python is passed we allow 2.7+ and 3.5+. In case nothing is passed we try to find 3.5+ first and then fall back to 2.7+