elementary OS 6 Odin support

[domagalhaes]

I'm using elementary OS 6 (Odin) based on Ubuntu 20.04 LTS and changed some of your code to support this distro:

• Added a release entry in /backends/elementary.py
• Added support for https and mirror protocols in ./init.py

It's working fine for me. I'll submit a pull request so you can analyze if there's impact on different distros.