Issue #18408: Fix dict_repr(), don't call PyObject_Repr() with an exception set

PyObject_Repr() can removes the current exception. For example, module_repr()
calls PyErr_Clear() if calling loader.module_repr(mod) failed.
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 3243061..36c710e 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -1443,6 +1443,9 @@
         Py_INCREF(value);
         s = PyObject_Repr(key);
         PyUnicode_Append(&s, colon);
+        if (s == NULL)
+            goto Done;
+
         PyUnicode_AppendAndDel(&s, PyObject_Repr(value));
         Py_DECREF(key);
         Py_DECREF(value);