Subclassing and adding QVector3d causes infinite recursion

Subclassing a QVector3D and adding the subclass to the QVector causes a RuntimeError: maximum recursion depth exceeded while calling a Python object

Minimum test case:

>>> from PySide.QtGui import QVector3D
>>> class Vector(QVector3D):
... pass
...
>>> v1 = Vector(1,2,3)
>>> v2 = QVector3D(1,2,3)
>>> v1+v1
PySide.QtGui.QVector3D(2.000000, 4.000000, 6.000000)
>>> v2+v2
PySide.QtGui.QVector3D(2.000000, 4.000000, 6.000000)
>>> v1+v2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: maximum recursion depth exceeded while calling a Python object

See this bug in pyqtgraph for the original report:
https://bugs.launchpad.net/pyqtgraph/+bug/1223173