pythonPackages.psutil: disable impure cpu_freq test

Tries to reach into /sys/devices/system/cpu, which may fail on
non-NixOS.

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

@@ -30,6 +30,7 @@ buildPythonPackage rec {
     "disk_io_counters"
     "sensors_battery"
     "cpu_times"
+    "cpu_freq"
   ];
 
   buildInputs = lib.optionals stdenv.isDarwin [ darwin.IOKit ];