old_contents: import json
  try:
      JSONDecodeError = json.decoder.JSONDecodeError
  except AttributeError:  # pragma Python 3.5,3.6: no cover
      JSONDecodeError = ValueError
  
new_contents: import json
  try:
      # JSONDecodeError class is available since Python 3.5.x.
      JSONDecodeError = json.decoder.JSONDecodeError
  except AttributeError:  # pragma Python 3.5,3.6: no cover
      JSONDecodeError = ValueError
  
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