Move WebSocketProtocolError to the legacy package.

This ensures that it will be deprecated and removed.

Author: aaugustin

src/websockets/exceptions.py

@@ -404,16 +404,16 @@ class ProtocolError(WebSocketException):
     """
 
 
-WebSocketProtocolError = ProtocolError  # for backwards compatibility
-
-
 # When type checking, import non-deprecated aliases eagerly. Else, import on demand.
 if typing.TYPE_CHECKING:
     from .legacy.exceptions import InvalidMessage
+
+    WebSocketProtocolError = ProtocolError
 else:
     lazy_import(
         globals(),
         aliases={
             "InvalidMessage": ".legacy.exceptions",
+            "WebSocketProtocolError": ".legacy.exceptions",
         },
     )

src/websockets/legacy/exceptions.py

@@ -1,4 +1,7 @@
-from ..exceptions import InvalidHandshake
+from ..exceptions import (
+    InvalidHandshake,
+    ProtocolError as WebSocketProtocolError,  # noqa: F401
+)
 
 
 class InvalidMessage(InvalidHandshake):