I'm seeing similar behaviour on jessie-backports:
# apt-get install 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:
letsencrypt : Depends: python-letsencrypt (= 0.4.1-1~bpo8+1) but it is
not going to be installed
E: Unable to correct problems, you have held broken packages.
# 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 (>= 0.4.1) but it is not
going to be installed
Depends: python-cryptography (>= 0.7) but 0.6.1-1
is to be installed
E: Unable to correct problems, you have held broken packages.
# apt-get install python-acme
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-acme : Depends: python-openssl (>= 0.15) but 0.14-1 is to be
installed
Depends: python-cryptography (>= 0.8) but 0.6.1-1 is to
be installed
E: Unable to correct problems, you have held broken packages.
