fix: Make releasable source package

Based on https://discourse.nixos.org/t/how-to-build-python-package-for-use-in-non-nix-environments/21587.

derivation.nix

 {pkgs}: let
+  pname = "vcard";
+  version = "0.15.0";
+
   python = pkgs.callPackage ./python.nix {};
-in
-  python.pkgs.buildPythonPackage {
-    pname = "vcard";
-    version = "0.15.0";
+
+  sdist = pkgs.stdenv.mkDerivation {
+    name = "${pname}-sdist";
 
     src = builtins.path {
+      name = pname;
       path = ./.;
-      name = "vcard";
     };
 
+    nativeBuildInputs = [
+      python
+      python.pkgs.setuptools
+    ];
+
+    buildPhase = ":";
+
+    installPhase = ''
+      python setup.py sdist --dist-dir="$out"
+    '';
+
+    strictDeps = true;
+  };
+in
+  python.pkgs.buildPythonPackage {
+    inherit pname version;
+
+    src = "${sdist}/${pname}*";
+
     propagatedBuildInputs = [
       python.pkgs.python-dateutil
       python.pkgs.typing-extensions
@@ -19,6 +40,12 @@ in
       python.pkgs.mock
     ];
 
+    pythonImportsCheck = ["vcard"];
+
+    postCheck = ''
+      gunzip --test dist/${pname}-${version}.tar.gz
+    '';
+
     meta = {
       pkgs.lib.homepage = "https://gitlab.com/victor-engmark/vcard";
       pkgs.lib.downloadPage = "https://pypi.org/project/vcard/";
@@ -29,4 +56,6 @@ in
       ];
       pkgs.lib.license = pkgs.lib.licenses.agpl3Plus;
     };
+
+    passthru.sdist = sdist;
   }