Use a better exception type.

src/websockets/legacy/protocol.py

@@ -697,7 +697,7 @@ async def ping(self, data: Optional[Data] = None) -> Awaitable[None]:
 
         :raises ~websockets.exceptions.ConnectionClosed: when the
             connection is closed
-        :raises ValueError: if another ping was sent with the same data and
+        :raises RuntimeError: if another ping was sent with the same data and
             the corresponding pong wasn't received yet
 
         """
@@ -708,7 +708,7 @@ async def ping(self, data: Optional[Data] = None) -> Awaitable[None]:
 
         # Protect against duplicates if a payload is explicitly set.
         if data in self.pings:
-            raise ValueError("already waiting for a pong with the same data")
+            raise RuntimeError("already waiting for a pong with the same data")
 
         # Generate a unique random payload otherwise.
         while data is None or data in self.pings:

tests/legacy/test_protocol.py

@@ -1021,7 +1021,7 @@ def test_canceled_ping(self):
     def test_duplicate_ping(self):
         self.loop.run_until_complete(self.protocol.ping(b"foobar"))
         self.assertOneFrameSent(True, OP_PING, b"foobar")
-        with self.assertRaises(ValueError):
+        with self.assertRaises(RuntimeError):
             self.loop.run_until_complete(self.protocol.ping(b"foobar"))
         self.assertNoFrameSent()