Tellstick install on Raspberry Pi

There is no signed version of Tellstick in newer version of raspian. Are you using stretch?
I think i run a raspian Jessie version. You can download older raspian version here Index of /raspbian/images

And if you run your HASS on a virtual enviroment the Python version can be upgrade. 3.6 version i minimum from next year.

Follow this guide till upgrade Python version in a VE.