python38Packages.flask_testing: fix build on darwin

pkgs/development/python-modules/flask-testing/default.nix

@@ -1,7 +1,4 @@
-{ stdenv, fetchPypi, buildPythonPackage, pythonOlder
-, flask, blinker, twill }:
-
-with stdenv.lib;
+{ stdenv, fetchPypi, buildPythonPackage, isPy3k, flask, blinker, twill }:
 
 buildPythonPackage rec {
   pname = "Flask-Testing";
@@ -13,13 +10,20 @@ buildPythonPackage rec {
   };
 
   postPatch = ''
-    sed -i -e 's/twill==0.9.1/twill/' setup.py
+    substituteInPlace setup.py --replace "twill==0.9.1" "twill"
   '';
 
-  buildInputs = optionals (pythonOlder "3.0") [ twill ];
-  propagatedBuildInputs = [ flask blinker ];
+  propagatedBuildInputs = [ flask ];
+
+  checkInputs = [ blinker ] ++ stdenv.lib.optionals (!isPy3k) [ twill ];
+
+  # twill integration is outdated in Python 2, hence it the tests fails.
+  # Some of the tests use localhost networking on darwin.
+  doCheck = isPy3k && !stdenv.isDarwin;
+
+  pythonImportsCheck = [ "flask_testing" ];
 
-  meta = {
+  meta = with stdenv.lib; {
     description = "Flask unittest integration.";
     homepage = "https://pythonhosted.org/Flask-Testing/";
     license = licenses.bsd3;