Require Python 3.4

Require Python 3.4

I believe we previously agreed that the minimum supported Python
version should be 3.4.  This patch makes this change, harmonizing the
documentation (which was inconsistent about the minimum version) and
the code.

New in v2: rebased, and removed a pre-3.4 workaround from __init__.py.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-by: Kevin Buettner <kevinb@redhat.com>
Acked-By: Tom de Vries <tdevries@suse.de>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31870