qipy: fix ./python script

Using exec is required for qi::path magic to work

Change-Id: Ie8083ead3556d690374e9fbb32b0763eb5a1d79e
Reviewed-on: http://gerrit.aldebaran.lan/48395
Tested-by: gerrit
Reviewed-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>
Tested-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>