chore: add nix flake and test for development

flake.nix

@@ -0,0 +1,26 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+  };
+
+  outputs = inputs@{ nixpkgs, flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = [
+        "x86_64-linux"
+      ];
+
+      perSystem = { self', system, pkgs, ... }: let
+        # common dependencies
+        deps = with pkgs.python3Packages; [
+          i3ipc
+        ];
+      in {
+        devShells = {
+          default = pkgs.mkShell {
+            packages = deps;
+          };
+        };
+      };
+    };
+}

test/run.py

@@ -0,0 +1,10 @@
+#!/usr/bin/env python
+
+import sys
+
+# use local package
+sys.path.insert(0, '..')
+
+from i3_focus_group.main import main
+
+main()