Motivation for this change

A few days ago, when updating all of my packages, I encountered build errors for khal / vdirsyncer mostly due to failing tests. When quickly patching an overlay to fix that, I noticed a few bad things in khal's default.nix:

1. pkgs is an argument to the derivation. This is not good because:
   • It's harder to change an input used inside khal such as pkgs.vdirsyncer. You have to use:

khal = super.khal.override {
  pkgs = pkgs // {
    vdirsyncer = vdirsyncer.override { ... };
  };
}

• I think that If you override another derivation via another overlay, that doesn't apply to what passed to that pkgs argument.

2. There's a with python3.pkgs; at the top - which makes it hard to derive python deps and non python deps (readability).
3. pkgs.vdirsyncer was used in propagatedBuildInputs. This is bad because:
   • it is required only for tests and although used frequently together along with khal, it isn't strictly required by it to run.
   • It increases closure size if somebody uses vdirsyncerStable.

Now, on master (without this PR) the tests don't fail but I strongly think this change is beneficial, for general maintainability and conformism with other unwritten conventions in Nixpkgs.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.