def start_shell(local_ns: Dict=None, banner: str=''):
    """Create and immediately drop into a Python shell.

    If IPython version 5 or greater is available it will be used instead
    of the built-in python shell.

    :param local_ns: An optional dict containing the global namespace of
                       the newly created shell.
    :param banner: An optional banner to render when terminal starts.
    """
    if IPYTHON_SHELL_AVAILABLE:
        # Don't try to stop IPython from displaying its banner, since
        # it's different in every major version
        terminal = embed.InteractiveShellEmbed(user_ns={})
        terminal.mainloop(local_ns=local_ns)
    else:
        code.interact(banner=banner, local=local_ns)