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PRETTY_NAME="Raspbian GNU/Linux 8 (jessie)"
NAME="Raspbian GNU/Linux"
VERSION_ID="8"
VERSION="8 (jessie)"

May be try this to force reinstall of pycrypto:
sudo apt-get install libffi-dev libssl-dev
pip3 install --user cryptography --force-reinstall

1 upgraded, 3 newly installed, 0 to remove and 134 not upgraded.

Do you want to continue? [Y/n] y
Abort.

after that still same error when running "piston addkey"

Any suggestions ?