Fix cell_contents (RustPython#5822)

Author: youknowone

Lib/test/test_funcattrs.py

@@ -140,8 +140,6 @@ def f(): print(a)
             self.fail("shouldn't be able to read an empty cell")
         a = 12
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_set_cell(self):
         a = 12
         def f(): return a

vm/src/builtins/function.rs

@@ -759,8 +759,11 @@ impl PyCell {
             .ok_or_else(|| vm.new_value_error("Cell is empty".to_owned()))
     }
     #[pygetset(setter)]
-    fn set_cell_contents(&self, x: PyObjectRef) {
-        self.set(Some(x))
+    fn set_cell_contents(&self, x: PySetterValue) {
+        match x {
+            PySetterValue::Assign(value) => self.set(Some(value)),
+            PySetterValue::Delete => self.set(None),
+        }
     }
 }