PyQt 4.10: use self._document = self.document()

in PygmentsHighlighter, required for PyQt 4.10 compatibility.

closes #3084

I have tested this on all my machines with earlier PyQt and PySide, and it doesn't break anything.  I don't have any truly old versions of PyQt, though.

see [PyQt list](http://www.riverbankcomputing.com/pipermail/pyqt/2013-March/032512.html) for details.