E: Failed to fetch https://deb.debian.org/debian-security/dists/stretch/updates/main/binary-amd64/Packages Received HTTP code 403 from proxy after CONNECT
To be expected:
"tell-me-what-you-need method" mentioned there would help.
This makes also enabling https (SSL / TLS) in anon-apt-soures-list (/etc/apt/sources.list.d/debian.list) difficult.
"tell-me-what-you-need method" is hard to add in /etc/apt/sources.list.d/debian.list but it is being used during build of Whonix.