nodes: assignments are not pure if the value is not None

compiler/nodes.py

@@ -400,6 +400,10 @@ class Assignment(Node):
     def _values(self) -> list[Node | Any]:
         return [self.identifier, self.value]
 
+    @property
+    def pure(self) -> bool:
+        return self.value is None
+
     def semantic_analysis(self, context: semantic.Context):
         super(Assignment, self).semantic_analysis(context)
         name = self.identifier.value