doc: PySide does not work in Python 3 ATM
remove import of pythonqt_workarounds

(Never was in this repository, and appears to be too
PDFDecanter-specific to be put here.)
re-enable the use of a C++ extension for PythonQt

PythonQt does not allow adding attributes to C++ objects, so even if the setup of the ndarray works correctly, the __array_interface__ trick does not.