FeatureNotFound( bs4.FeatureNotFound: Couldn‘t find a tree builder with the features you requested:

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