问题描述：
代码如下：
from bs4 import BeautifulSoup
# 在此实现代码
def fetch_text(html):
soup = BeautifulSoup(html, 'lxml')
result = soup.text
return result
执行到这行：BeautifulSoup(html, ‘lxml’)，报如下错误：
in init
raise FeatureNotFound(
bs4.FeatureNotFound: Couldn’t find a tree builder with the features you requested: lxml. Do you need to install a parser library?
执行如下命令，下载安装lxml:
pip install lxml
