iotests: Require Python 3.6 or later

iotests: Require Python 3.6 or later

Running iotests is not required to build QEMU, so we can have stricter
version requirements for Python here and can make use of new features
and drop compatibility code earlier.

This makes qemu-iotests skip all Python tests if a Python version before
3.6 is used for the build.

Suggested-by: Eduardo Habkost <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>