Add protected fields to PlainBlock so NodeTransformer knows it's a composite node

trans/transpiler/phases/utils.py

@@ -31,6 +31,8 @@ class NodeVisitorSeq:
 @dataclass
 class PlainBlock(ast.stmt):
     body: list[ast.stmt]
+    _fields = ("body",)
+    __match_args__ = ("body",)
 
 
 @dataclass