FIX: define DataFrame.items for all versions of python

Closes #17213

This leaves a slight semantic difference between `dict.items` and
`DateFrame.items` in python2, however there is no code currently
expecting `DataFrame.items` to return a list and in python3 there is
no `dict.iteritems`.

This eases writing 'native' python3 code that also runs under python2.

Author: tacaswell

Diff for: pandas/core/frame.py

@@ -802,8 +802,7 @@ def itertuples(self, index=True, name="Pandas"):
         # fallback to regular tuples
         return zip(*arrays)
 
-    if compat.PY3:  # pragma: no cover
-        items = iteritems
+    items = iteritems
 
     def __len__(self):
         """Returns length of info axis, but here we use the index """

Diff for: pandas/tests/frame/test_api.py

@@ -164,6 +164,11 @@ def test_iteritems(self):
         for k, v in compat.iteritems(df):
             assert type(v) == Series
 
+    def test_items(self):
+        df = DataFrame([[1, 2, 3], [4, 5, 6]], columns=['a', 'a', 'b'])
+        for k, v in df.items():
+            assert type(v) == Series
+
     def test_iter(self):
         assert tm.equalContents(list(self.frame), self.frame.columns)