use self._document = self.document()

in PygmentsHighlighter, for PyQt 4.10 compatibility

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

closes #3084
Merge pull request #3097 from minrk/qt-4.10

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