Ошибка источника top.postMessage не обнаружена

Вы можете использовать fileinput для этого:

import fileinput
import sys
import os

inputFileName = "temp.csv"
outputFileName = os.path.splitext(inputFileName)[0] + "_modified.csv"

with open(outputFileName, "w") as outfile:
    for line in fileinput.input(
        [inputFileName],
        inplace=False):
        if fileinput.isfirstline():
            outfile.write('Item Number,Item Description,List Price,QTY Available\n')
        else:
            outfile.write(line)