iotests: Remove Python 2 compatibility code

iotests: Remove Python 2 compatibility code

Some scripts check the Python version number and have two code paths to
accomodate both Python 2 and 3. Remove the code specific to Python 2 and
assert the minimum version of 3.6 instead (check skips Python tests in
this case, so the assertion would only ever trigger if a Python script
is executed manually).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>