docs: correct outdated MappingProxyType docstrings (GH-30281)

The docstrings for MappingProxyType's keys(), values(), and items()
methods were never updated to reflect the changes that Python 3 brought
to these APIs, namely returning views rather than lists.
(cherry picked from commit 2d10fa9)

Co-authored-by: Joshua Bronson <jabronson@gmail.com>

Author: miss-islington

Objects/descrobject.c

@@ -1135,11 +1135,11 @@ static PyMethodDef mappingproxy_methods[] = {
      PyDoc_STR("D.get(k[,d]) -> D[k] if k in D, else d."
                "  d defaults to None.")},
     {"keys",      (PyCFunction)mappingproxy_keys,       METH_NOARGS,
-     PyDoc_STR("D.keys() -> list of D's keys")},
+     PyDoc_STR("D.keys() -> a set-like object providing a view on D's keys")},
     {"values",    (PyCFunction)mappingproxy_values,     METH_NOARGS,
-     PyDoc_STR("D.values() -> list of D's values")},
+     PyDoc_STR("D.values() -> an object providing a view on D's values")},
     {"items",     (PyCFunction)mappingproxy_items,      METH_NOARGS,
-     PyDoc_STR("D.items() -> list of D's (key, value) pairs, as 2-tuples")},
+     PyDoc_STR("D.items() -> a set-like object providing a view on D's items")},
     {"copy",      (PyCFunction)mappingproxy_copy,       METH_NOARGS,
      PyDoc_STR("D.copy() -> a shallow copy of D")},
     {"__class_getitem__", (PyCFunction)Py_GenericAlias, METH_O|METH_CLASS,