QTPY py3 translation

[cgvirus]

There are linking issues in with 2.5.0 Alpha 2 still.
NatronGitHub/Natron#701 (comment)

But should work without any problem in 2.4.3.
Plugs are compatible with Pyhton 3. Also an import exception handling is included:

try:
    from qtpy.QtGui import *
except ImportError:
    from PySide.QtGui import *