🐛 Fix mypy [type-arg] error by giving Data TypeVar a default of Any

Bare `ServerSentEvent` (without a type parameter) now silently resolves
to `ServerSentEvent[Any]`, so existing code and the `_check_data_exclusive`
return annotation require no changes and pass mypy without `[type-arg]`
errors.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

fastapi/sse.py

@@ -1,10 +1,12 @@
-from typing import Annotated, Any, Generic, TypeVar
+from typing import Annotated, Any, Generic
+
+from typing_extensions import TypeVar
 
 from annotated_doc import Doc
 from pydantic import AfterValidator, BaseModel, ConfigDict, Field, model_validator
 from starlette.responses import StreamingResponse
 
-Data = TypeVar("Data")
+Data = TypeVar("Data", default=Any)
 """Type variable for the `data` payload of a `ServerSentEvent`.
 
 Use ``ServerSentEvent[MyModel]`` to indicate that every event in the