Fix inheritance diagram generation

Inheritance diagram generation currently does not work. sources/pyside2/doc/inheritance_diagram.py complains (line 100)

 WARNING: Could not import class 'QXmlName' specified for inheritance diagram

When changing that py passing the fully qualified name in QtDocGenerator::generateClass() at sources/shiboken2/generator/qtdoc/qtdocgenerator.cpp:1326

s << ".. inheritance-diagram:: " << className << endl
-->
s << ".. inheritance-diagram:: " << metaClass->fullName() << endl

an exception is thrown by sphinx:

TypeError: can't pickle dict_values objects

First happens in QtConcurrent, but also seems to affect QtCore classes.

The invocation is sth like

sphinx-build -b html  <build>/py3.6-qt5.11.0-64bit-release/pyside2/doc/rst html