bpo-28280: Make PyMapping_Keys(), PyMapping_Values() and PyMapping_Items() always return a list (#3840)

diff --git a/Doc/c-api/mapping.rst b/Doc/c-api/mapping.rst
index a71e942..308a976 100644
--- a/Doc/c-api/mapping.rst
+++ b/Doc/c-api/mapping.rst
@@ -50,20 +50,29 @@
 
 .. c:function:: PyObject* PyMapping_Keys(PyObject *o)
 
-   On success, return a list or tuple of the keys in object *o*.  On failure,
-   return *NULL*.
+   On success, return a list of the keys in object *o*.  On failure, return
+   *NULL*.
+
+   .. versionchanged:: 3.7
+      Previously, the function returned a list or a tuple.
 
 
 .. c:function:: PyObject* PyMapping_Values(PyObject *o)
 
-   On success, return a list or tuple of the values in object *o*.  On failure,
-   return *NULL*.
+   On success, return a list of the values in object *o*.  On failure, return
+   *NULL*.
+
+   .. versionchanged:: 3.7
+      Previously, the function returned a list or a tuple.
 
 
 .. c:function:: PyObject* PyMapping_Items(PyObject *o)
 
-   On success, return a list or tuple of the items in object *o*, where each item
-   is a tuple containing a key-value pair.  On failure, return *NULL*.
+   On success, return a list of the items in object *o*, where each item is a
+   tuple containing a key-value pair.  On failure, return *NULL*.
+
+   .. versionchanged:: 3.7
+      Previously, the function returned a list or a tuple.
 
 
 .. c:function:: PyObject* PyMapping_GetItemString(PyObject *o, const char *key)
diff --git a/Doc/whatsnew/3.7.rst b/Doc/whatsnew/3.7.rst
index ecdd2fe..24a1dc4 100644
--- a/Doc/whatsnew/3.7.rst
+++ b/Doc/whatsnew/3.7.rst
@@ -404,6 +404,10 @@
   is now of type ``const char *`` rather of ``char *``. (Contributed by Serhiy
   Storchaka in :issue:`28769`.)
 
+* The result of :c:func:`PyMapping_Keys`, :c:func:`PyMapping_Values` and
+  :c:func:`PyMapping_Items` is now always a list, rather than a list or a
+  tuple. (Contributed by Oren Milman in :issue:`28280`.)
+
 * Added functions :c:func:`PySlice_Unpack` and :c:func:`PySlice_AdjustIndices`.
   (Contributed by Serhiy Storchaka in :issue:`27867`.)