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/import.c b/Python/import.c
index 8a948b9..036d17e 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -2691,8 +2691,8 @@
 		parent = PyDict_GetItem(modules, parentname);
 		if (parent == NULL) {
 			PyErr_Format(PyExc_ImportError,
-			    "reload(): parent %.200s not in sys.modules",
-			     _PyUnicode_AsString(parentname));
+			    "reload(): parent %U not in sys.modules",
+			     parentname);
 			Py_DECREF(parentname);
 			imp_modules_reloading_clear();
 			return NULL;