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 <[email protected]>
Message-Id: <20180608143026[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Cleber Rosa <[email protected]>
Reviewed-by: John Snow <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>