Get the filenames in `path/csv_name` opened with `header`.

def from_csv(cls, path:PathOrStr, csv_name:str, header:str='infer', **kwargs)->'ItemList':
        "Get the filenames in `path/csv_name` opened with `header`."
        path = Path(path)
        df = pd.read_csv(path/csv_name, header=header)
        return cls.from_df(df, path=path, **kwargs)