bs4.FeatureNotFound: Couldn’t find a tree builder with the features you requested: lxml.

python3 bs4解析网页时报错： bs4.FeatureNotFound: Couldn’t find a tree builder with the features you requested: lxml.

bs4调用了python自带的html解析器，python3没有html解释器，所以会报错。
给python3装一个html解析器：
pip3 install lxml