In addition to the added packages, it is necessary to install lxml within the virtual environment if AiO install has been chosen (I did not verify with other installs). Otherwise HASS will not start and not give any error messages when device_tracker with Fritz is used.
Without it fritzconnection might not be able to install correctly on low-end machines (e.g. Raspberry Pi) due to running out of resources while trying to compile lxml via pip3.