Nixpkgs: Octoprint broken due to Python 2.7 EOL

[WhittlesJr]

Describe the bug
Similar to #71940, but probably more pernicious as Django is still a requirement:

Django-2.2.10 not supported for interpreter python2.7

The recent Octoprint 1.4.0 release supports Python 3. Update to Octoprint 1.4.0 (non-trivial, but WIP PR attached. I need some help with this one.)

To Reproduce
Steps to reproduce the behavior:

1. add services.services.octoprint.enable = true to your config
2. nixos-rebuild switch

Expected behavior
Octoprint builds

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.22, NixOS, 20.09pre214776.ca353185084 (Nightingale)`
 - multi-user?: `no`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.3`
 - channels(root): `"nixos-20.09pre214776.ca353185084"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

[gebner]

Fixed by #82392, which upgrades octoprint to 1.4.0 and python 3.