Python: use self as default _root only in top-level types

... as in Java, see https://github.com/kaitai-io/kaitai_struct_compiler/blob/829a14f1e33e8e48eeae726c8a287a5967bcb668/shared/src/main/scala/io/kaitai/struct/languages/JavaCompiler.scala#L153

This commit was extracted from the `serialization` branch (originally
e776c98), see
#255

shared/src/main/scala/io/kaitai/struct/languages/PythonCompiler.scala

@@ -101,7 +101,11 @@ class PythonCompiler(typeProvider: ClassTypeProvider, config: RuntimeConfig)
     out.inc
     out.puts("self._io = _io")
     out.puts("self._parent = _parent")
-    out.puts("self._root = _root if _root else self")
+    if (name == rootClassName) {
+      out.puts("self._root = _root if _root else self")
+    } else {
+      out.puts("self._root = _root")
+    }
 
     if (isHybrid)
       out.puts("self._is_le = _is_le")