python3Packages.cliff: fix build and add tests

pkgs/development/python-modules/cliff/default.nix

@@ -9,6 +9,10 @@
 , pyyaml
 , unicodecsv
 , cmd2
+, pytest
+, mock
+, testtools
+, fixtures
 }:
 
 buildPythonPackage rec {
@@ -31,10 +35,17 @@ buildPythonPackage rec {
     unicodecsv
   ];
 
-  # test dependencies are complex
-  # and would require about 20 packages
-  # to be added
-  doCheck = false;
+  # remove version constraints
+  postPatch = ''
+    sed -i '/cmd2/c\cmd2' requirements.txt
+  '';
+
+  checkInputs = [ fixtures mock pytest testtools ];
+  # add some tests
+  checkPhase = ''
+    pytest cliff/tests/test_{utils,app,command,help,lister}.py \
+      -k 'not interactive_mode'
+  '';
 
   meta = with lib; {
     description = "Command Line Interface Formulation Framework";