fix union annotation for async file proto

Author: tsimoshka

httpx/_types.py

@@ -84,7 +84,7 @@ async def read(self, size: int = -1) -> AnyStr: ...
 
     def fileno(self) -> int: ...
 
-    async def seek(self, offset: int, whence: int | None = ...) -> int: ...
+    async def seek(self, offset: int, whence: Optional[int] = ...) -> int: ...
 
 
 FileContent = Union[IO[bytes], bytes, str, AsyncReadableBinaryFile]