Fix Python shebang in examples subpackage to avoid depending on Python2
    
    The "/usr/bin/env python" shebang in maliit-exampleapp-gtk3-python.py executable is being mangled during build to "/usr/bin/python2" and is generating a dependency on Python 2. This PR replaces it with "/usr/bin/python3" so that the subpackage requires Python 3 only.