Quick build fix while waiting for PyQt6-6.8.2, not entirely sure
if this is correct as hardly know sip.
--- a/sip/QtCore/qstringconverter_base.sip
+++ b/sip/QtCore/qstringconverter_base.sip
@@ -24,3 +24,3 @@
 
-class QStringConverterBase
+class QStringConverterBase /NoDefaultCtors/
 {