switch from partial to direct method call for TogaSlider

iOS/src/toga_iOS/widgets/slider.py

@@ -81,7 +81,7 @@ def get_value(self):
 
     def set_value(self, value):
         self.value = value
-        self.native.setValue(value, animated=True)
+        self.native.setValue_animated_(value, True)
 
     def get_min(self):
         # Use the shadow copy, not the native value, to ensure round tripping.