python3Packages.pip: add pip-tools to tests

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

@@ -7,6 +7,9 @@
 , virtualenv
 , pretend
 , pytest
+
+# coupled downsteam dependencies
+, pip-tools
 }:
 
 buildPythonPackage rec {
@@ -32,6 +35,8 @@ buildPythonPackage rec {
   # Pip wants pytest, but tests are not distributed
   doCheck = false;
 
+  passthru.tests = { inherit pip-tools; };
+
   meta = {
     description = "The PyPA recommended tool for installing Python packages";
     license = with lib.licenses; [ mit ];