with open('file.txt', 'r') as old_file:
  with open('file.txt', 'r+') as new_file:

    current_line = 0

    # 定位到需要删除的行
    while current_line < (3 - 1): #(del_line - 1)
      old_file.readline()
      current_line += 1

    # 当前光标在被删除行的行首，记录该位置
    seek_point = old_file.tell()

    # 设置光标位置
    new_file.seek(seek_point, 0)

    # 读需要删除的行，光标移到下一行行首
    old_file.readline()

    # 被删除行的下一行读给 next_line
    next_line = old_file.readline()

    # 连续覆盖剩余行，后面所有行上移一行
    while next_line:
      new_file.write(next_line)
      next_line = old_file.readline()

    # 写完最后一行后截断文件，因为删除操作，文件整体少了一行，原文件最后一行需要去掉
    new_file.truncate()