pythonPackages: fix native / check inputs

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

@@ -12,7 +12,7 @@ if isPyPy then null else buildPythonPackage rec {
   outputs = [ "out" "dev" ];
 
   propagatedBuildInputs = [ libffi pycparser ];
-  buildInputs = [ pytest ];
+  checkInputs = [ pytest ];
 
   # On Darwin, the cffi tests want to hit libm a lot, and look for it in a global
   # impure search path. It's obnoxious how much repetition there is, and how difficult