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

All modified and coverable lines are covered by tests ✅

Project coverage is 63.12%. Comparing base (17db314) to head (9a92e71).
Report is 176 commits behind head on next.

Additional details and impacted files

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

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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Also this fork is very outdated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. 😎

[Mast3rwaf1z]

@krestenlaust with #459 merged, I think we should close this pr, it basically does the same with less flexibility

[krestenlaust]

Fixed by #459