py/objdict: Fix popitem for ordered dicts.

The popitem method wasn't implemented for ordered dicts and would result in
an invalid state.

Fixes issue #5956.

tests/basics/ordereddict1.py

@@ -24,3 +24,23 @@ d["abc"] = 123
 print(len(d))
 print(list(d.keys()))
 print(list(d.values()))
+
+# pop an element
+print(d.popitem())
+print(len(d))
+print(list(d.keys()))
+print(list(d.values()))
+
+# add an element after popping
+d["xyz"] = 321
+print(len(d))
+print(list(d.keys()))
+print(list(d.values()))
+
+# pop until empty
+print(d.popitem())
+print(d.popitem())
+try:
+    d.popitem()
+except:
+    print('empty')