使用requests实现下载进度条
    流: data_response = requests.get(url, stream=True)
    文件长度: file_size = requests.head(url).headers.get('Content-Length')
    分块: for block in data_response.iter_content(1024) #
    每段最大1024，实际len(block)
    进度: sys.stdout.write() + \r + sys.stdout.flush()