fix CPython compatibility problem on lookup failure in optimised PyDict_GetItem()

Cython/Utility/ObjectHandling.c

@@ -221,8 +221,12 @@ static PyObject *__Pyx_PyDict_GetItem(PyObject *d, PyObject* key) {
     PyObject *value;
     value = PyDict_GetItemWithError(d, key);
     if (unlikely(!value)) {
-        if (!PyErr_Occurred())
-            PyErr_SetObject(PyExc_KeyError, key);
+        if (!PyErr_Occurred()) {
+            PyObject* args = PyTuple_Pack(1, key);
+            if (likely(args))
+                PyErr_SetObject(PyExc_KeyError, args);
+            Py_XDECREF(args);
+        }
         return NULL;
     }
     Py_INCREF(value);

tests/run/dict_getitem.pyx

@@ -17,6 +17,22 @@ def test(dict d, index):
     Traceback (most recent call last):
     KeyError: (1, 2)
 
+    >>> import sys
+    >>> try: d[(1,)]
+    ... except KeyError:
+    ...     args = sys.exc_info()[1].args
+    ...     if sys.version_info >= (2,5): print(args)
+    ...     else: print((args,))   # fake it for older CPython versions
+    ((1,),)
+
+    >>> import sys
+    >>> try: test(d, (1,))
+    ... except KeyError:
+    ...     args = sys.exc_info()[1].args
+    ...     if sys.version_info >= (2,5): print(args)
+    ...     else: print((args,))   # fake it for older CPython versions
+    ((1,),)
+
     >>> class Unhashable:
     ...    def __hash__(self):
     ...        raise ValueError