Pip install bs4 失败

The repository located at pypi.douban.com is not a trusted or secure host and is being ignored. If this repository is available via HTTPS it is recommended to use HTTPS instead, otherwise you may silence this warning and allow it anyways with '--trusted-host pypi.douban.com'.
  Could not find a version that satisfies the requirement bs4 (from versions: )
No matching distribution found for bs4
You are using pip version 9.0.3, however version 10.0.0 is available.

You should consider upgrading via the 'pip install --upgrade pip' command.

解决方案：

pip3 --trusted-host pypi.douban.com install bs4