Package/qscintilla: python bindings via extend path (#15599)

* build python bindings within qscintilla package via extend_path trick

* add todo

* reflect new setup also in py-pyqt4 package

* get rid of qscintilla dependency

* also tweak qgis for the new setup

* generalize the building of python bindings

* generalize building of pythong bindings to all qt versions

* add qsci_api variant

* add qsci_variant for pyqt4 package as well; add comment

* pyqt dependency should build with +qsci_api variant enabled

* fix bugs

* improve style

* reflect recent changes

* flake8

* improve style

* more flake8

* more flake8

Co-authored-by: Sinan81 <sbulut@3vgeomatics.com>