[RFC] build: Drop Python 2 support and require Python 3.4+
See https://mail.gnome.org/archives/desktop-devel-list/2018-July/msg00004.html for a discussion on if/when we can start relying on Python 3 only.
Use Python 3.4 as a new requirement because that's the version used in SLES 12 and Debian 8 and there is no good reason to require something newer right now.