code: def _parse_downloadpage_html(self, doc):
          
          soup = bs4.BeautifulSoup(doc, 'lxml')
          a = soup.select('.download-box > a.btn-click')
          if a:
              a = a[0]
              link = a.get('href')
              return link
          return ''
docs: 解析下载页面，返回下载链接