PyQt: Update sip to 6.5.0, reintroduce sip4 and simplify PyQt framework.

SIP:
As mentioned in the update from sip to sip5, this is a transitional version
to remove what is deprecated in sip4.
Sip6 completely removes the deprecated parts.
Unfortunately, some ports — mostly cura things — can not use sip6, so we
reintroduce sip4.

PyQt:
At the same time, we took the opportunity to simplify PyQT and propose only one
package as for devel/pyside2. */py-qt5-* have been merged — excepted chart,
networkauth and webengine — into devel/py-qt5-pyqt.

This allows us to be in adequacy with the packages that the author of these
libraries proposes, namely:

PyQt - devel/py-qt5-pyqt
PyQt-Charts - x11-toolkits/py-qt5-chart
PyQt-NetworkAuth – net/py-qt5-networkauth
PyQt-WebEngine – www/py-qt5-webengine
SIP – devel/py-sip
py-sip - devel/py-qt5-sip
PyQt-builder - devel/py-qtbuilder
Qscintilla - devel/py-qt5-qscintilla2

Reviewed by: diizzy, kde
Tested by: kai, rhurlin, arrowd, madpilot
Approved by: makc, tcberner, kde
Differential Revision: https://reviews.freebsd.org/D33237
Exp-run by: antoine