Update Qt/PySide2 to 5.15.2 / PyQt to 5.15.1 / SIP to 5.4.0
This commit combines several updates.

• Update Qt to 5.12.2
• Update PyQt to 5.15.1
• Since PySide 5.15.1 is broken with Qt 5.15.2, so PySide and Shiboken are also updated to 5.15.2.
• Update sip to 5.4.0

SIP is a collection of tools to create Python bindings for C and C++ libraries and used by PyQt and wxPython. There are some changes with sip5 [1]:

• python 3.5+ is required
• sip drops support of old deprecated methods as sipdistutils & cie.
• this version breaks also PyQt5 extension ABI. SIP files will be installed in ${PYTHON_SITELIBDIR}/PyQt${_PYQT_VERSION}/bindings
• some ports will use the new pyqtbuilder package with the pyproject.toml setup. Instead if the project have a setup.py, you should use sip-build to build it.
• if a port needs sip, it should also needs pysip. BTW, py-qt5-core requires pysip, so that should be enough for PyQt packages.

List of ports impacted by this change. Most of the patches have been integrated or are in the process of being integrated upstream:

• devel/libsavitar
• graphics/py-python-poppler-qt5
• net-im/scudcloud
• net/libarcus
• print/py-frescobaldi
• science/py-veusz
• graphics/qgis and graphics/qgis-ltr
• deskutils/calibre

A special note regarding calibre. New versions require sip>=5, we update it to the latest version (thanks to madpilot@)

science/scidavis will remove the PyQt binding soon and there is no patch planned, so we remove the python option (ok makc@)

While here, convert some ports to USE_PYQT (cad/cura, cad/uranium, devel/eric6, ...)

Thanks to tcberner and adridg!

[1] https://www.riverbankcomputing.com/static/Docs/sip/

PR: 250853
Exp-run by: antoine