old_contents: from .checkpoints import PostgresCheckpoints
  from .pgmanager import PostgresContentsManager
  
  __all__ = [
      'PostgresCheckpoints',
      'PostgresContentsManager',
  ]
  
new_contents: # Do this first so that we bail early with a useful message if the user didn't
  # specify [ipy3] or [ipy4].
  try:
      import IPython  # noqa
  except ImportError:
      raise ImportError(
          "No IPython installation found.\n"
          "To install pgcontents with the latest Jupyter Notebook"
          " run 'pip install pgcontents[ipy4]b'.\n"
          "To install with the legacy IPython Notebook"
          " run 'pip install pgcontents[ipy3]'.\n"
      )
  
  from .checkpoints import PostgresCheckpoints
  from .pgmanager import PostgresContentsManager
  
  __all__ = [
      'PostgresCheckpoints',
      'PostgresContentsManager',
  ]
  
subject: Add warning if IPython isn't installed.