build: making dev shell with nix flake

Bibliography:
- https://nixos-and-flakes.thiscute.world/zh/development/dev-environments
- https://github.com/MordragT/nix-templates/blob/bb60675e0516d8a441dcb6f87559b7436977bb0c/python-venv/flake.nix
- https://nixos.org/manual/nixpkgs/unstable/#python

.gitignore

@@ -3,3 +3,6 @@ __pycache__/
 
 # Distribution / packaging
 dist/
+
+# Virtual environments
+.venv

flake.nix

@@ -0,0 +1,38 @@
+{
+  description = "SNNCutoff's development environment";
+
+  inputs = {
+    utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = {
+    self,
+    nixpkgs,
+    utils,
+    ...
+  }:
+    utils.lib.eachDefaultSystem (system: let
+      pkgs = import nixpkgs {
+        inherit system;
+      };
+      pypkgs = pkgs.python3Packages;
+    in {
+      devShells.default = pkgs.mkShell {
+        venvDir = "./.venv";
+        buildInputs = [
+          pypkgs.python
+          pypkgs.venvShellHook  # creates a venv in $venvDir
+        ];
+
+        postVenvCreation = ''
+          python -m pip install --editable .[dev]
+        '';
+
+        postShellHook = ''
+        '';
+
+        # Enable python libraries to discover libstdc++.so.6
+        LD_LIBRARY_PATH = with pkgs; lib.makeLibraryPath [ stdenv.cc.cc ];
+      };
+    });
+}