weblate: init at 5.6.2

[erictapen]

This adds the weblate package, module and integration test from my previous work at https://github.com/ngi-nix/weblate.

As this package attempt uses poetry2nix, which isn't part of Nixpkgs anymore, I had to add it as a Flake input to NgiPkgs.
Also I noticed that I would benefit from the snakeoil TLS certificates used by many NixOS tests in Nixpkgs, so I figured it would make sense to expose nixpkgs as part of sources to every project?

I also update the package to the latest version 5.6.2.

Let me know if I should squash for inclusion, the commit messages are more or less written for myself.

Edit: Pinging @fricklerhandwerk as he asked me to do this, and @lorenzleutgeb as I saw your earlier branch.

Closes #221

[erictapen]

Just noticed that the overview output auto-generates a URL to the project site: https://nlnet.nl/project/Weblate

But I can't find a Weblate project in https://nlnet.nl/project/? Is it just not listed maybe?

[fricklerhandwerk]

Oh, it may never have been funded by NLnet. I suppose we'll have to adjust how we do the attribution.

[erictapen]

I just opened NixOS/nixpkgs#325541 to have Weblate in Nixpkgs. If that succeeds, having Weblate in ngipkgs would probably only make sense until the next NixOS stable release.

[erictapen]

Had to fix an override for aeidon, which probably broke due to a Python upgrade.