configure: Require Python 2.7 or newer

configure: Require Python 2.7 or newer

All of the supported build platforms documented in qemu-doc.texi
should already support Python 2.7.

Removing support for Python 2.6 will allow us to remove some
compatibility modules we carry in the QEMU tree:

* scripts/argparse.py
* scripts/ordereddict.py

Python 2.6 is also not receiving bug fixes upstream and is not
supported by pylint, which makes it harder to keep the code
compatible with both Python 2 and Python 3.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20180608143026.20167-1-ehabkost@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>