swap branch for line in dtls

Author: graingert

src/trio/_dtls.py

@@ -1039,7 +1039,7 @@ def read_volley() -> list[_AnyHandshakeMessage]:
                             if (
                                 isinstance(maybe_volley[0], PseudoHandshakeMessage)
                                 and maybe_volley[0].content_type == ContentType.alert
-                            ):  # TODO: test this branch
+                            ):  # TODO: test this line
                                 # we're sending an alert (e.g. due to a corrupted
                                 # packet). We want to send it once, but don't save it to
                                 # retransmit -- keep the last volley as the current