fix: update qgis4 compatibility

Author: lachlangrose

loopstructural/gui/compatibility.py

@@ -1,7 +1,14 @@
 # compat.py
 from qgis.PyQt.QtCore import QVariant
 
-try:
+if hasattr(QVariant, "Int"):
+    # QGIS 3 / PyQt5: QVariant still exposes the legacy .Type enum members.
+    # (QMetaType is importable under PyQt5 too, so we can't use that import
+    # to detect QGIS 4 - checking for the attribute QGIS 4 actually removed
+    # is the only reliable signal.)
+    QVariantCompat = QVariant
+else:
+    # QGIS 4 / PyQt6: QVariant.Type was removed; use QMetaType instead.
     from qgis.PyQt.QtCore import QMetaType
 
     # We create a proxy class to mimic the old QVariant.Type behavior
@@ -12,9 +19,6 @@ class QVariantProxy:
         Double = QMetaType.Type.Double
         String = QMetaType.Type.QString
         Bool = QMetaType.Type.Bool
+        DateTime = QMetaType.Type.QDateTime
 
-    # In QGIS 4, we use our proxy
     QVariantCompat = QVariantProxy
-except (ImportError, AttributeError):
-    # In QGIS 3, QVariant already has .Type
-    QVariantCompat = QVariant