Issue #6697: use %U format instead of _PyUnicode_AsString(), because
_PyUnicode_AsString() was not checked for error (NULL).

The unicode string is no more truncated to 200 or 400 *bytes*.
diff --git a/Python/ceval.c b/Python/ceval.c
index 47c53cf..4e9f77b 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -3989,10 +3989,10 @@
 		if (PyDict_GetItem(kwdict, key) != NULL) {
 			PyErr_Format(PyExc_TypeError,
 				     "%.200s%s got multiple values "
-				     "for keyword argument '%.200s'",
+				     "for keyword argument '%U'",
 				     PyEval_GetFuncName(func),
 				     PyEval_GetFuncDesc(func),
-				     _PyUnicode_AsString(key));
+				     key);
 			Py_DECREF(key);
 			Py_DECREF(value);
 			Py_DECREF(kwdict);