Fix stub

Author: JukkaL

mypy/typeshed/stubs/librt/librt/internal.pyi

@@ -1,7 +1,7 @@
 from mypy_extensions import u8
 
 class ReadBuffer:
-    def __init__(self, source: bytes = ...) -> None: ...
+    def __init__(self, source: bytes) -> None: ...
 
 class WriteBuffer:
     def getvalue(self) -> bytes: ...