dpkg 抱怨缺少已安裝的軟體包

嘗試letsencrypt在Debian Jessie上安裝，我遇到了以下我不太明白的情況。我已經安裝了python-acme：

dpkg -l | grep acme
ii  python-acme  0.6.0-1~bpo8+1  all  ACME protocol library for Python 2

現在我正在嘗試安裝python-letsencrypt：

apt-get install python-letsencrypt
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
python-letsencrypt : Depends: python-acme (>= 0.5.0) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

這個錯誤資訊是什麼意思？據我所知，我安裝了正確版本的軟體包（0.6.0-1~bpo8+1是>= 0.5.0，對嗎？）為什麼軟體包管理器拒絕查看它？

我懷疑虛擬包的過渡和使用存在一些問題certbot……（直到最近，虛擬包不支持版本依賴項。）

由於certbot現在在 Jessie backports 中，並且 replaces letsencrypt，我建議安裝它：

apt-get install -t jessie-backports certbot

引用自：https://unix.stackexchange.com/questions/288816