Add nix-shell setup for easy development

[SEbbaDK]

The shell specification uses the current requirements.txt and should add no maintenance.
It makes it very simple for nix users on linux/mac to get ready to work on stregsystemet via nix-shell.
We can hopefully remove the two custom packages later when @jbjjbjjbj upstreams the packages to nixpkgs

[falkecarlsen]

I've heard it doesn't matter if the 'main' nix-file specifies py-version, but just for good order

[SEbbaDK]

@falkecarlsen I made the shell show a clear warning about the python-version being used, so hopefully the mismatch isn't as big of a deal.

[codecov-commenter]

Codecov Report

Merging #272 (9a92e71) into next (17db314) will decrease coverage by 19.35%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             next     #272       +/-   ##
===========================================
- Coverage   82.48%   63.12%   -19.36%     
===========================================
  Files          30       30               
  Lines        2329     3051      +722     
  Branches      167      212       +45     
===========================================
+ Hits         1921     1926        +5     
- Misses        379     1061      +682     
- Partials       29       64       +35     

Impacted Files	Coverage Δ
treo/settings.py	41.66% <0.00%> (-56.16%)	⬇️
treo/urls.py	35.29% <0.00%> (-31.38%)	⬇️
stregreport/forms.py	71.42% <0.00%> (-28.58%)	⬇️
stregsystem/utils.py	49.31% <0.00%> (-28.11%)	⬇️
stregsystem/mail.py	42.42% <0.00%> (-27.58%)	⬇️
kiosk/urls.py	75.00% <0.00%> (-25.00%)	⬇️
stregsystem/models.py	65.51% <0.00%> (-21.91%)	⬇️
stregsystem/deprecated.py	50.00% <0.00%> (-21.43%)	⬇️
stregreport/urls.py	80.00% <0.00%> (-20.00%)	⬇️
stregsystem/views.py	40.88% <0.00%> (-19.97%)	⬇️
... and 10 more

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[falkecarlsen]

I don't like that this PR inadvertently also changes which next py version we're targeting. Since Django 2.2.17, there has been support for py39, so it's not a problem.

[Mast3rwaf1z]

@krestenlaust it works with nixos-23.05, but later versions like 23.11 or unstable seems to have checks in place against the older version of django we use.

There seems to be a missing dependency to a module called django.utils.six, i don't know what this is.

Also this fork is very outdated

I would suggest that if we want a nix shell, i would prefer it to be written as a flake instead, and rewritten to work with the current system.

Edit: i do like how this is basing package versions on the requirements.txt tho

[krestenlaust]

Thanks you for the insight @Mast3rwaf1z 👌

[soupglasses]

Waiting for #409 so we dont need to package ancient versions of Python dependencies. 😎