python3Packages.maxminddb: fix build on Darwin

The multiprocessing tests don’t work on Darwin presumably because it
uses the spawn start method instead of fork. Disable those tests to
allow maxminddb to build on Darwin again.

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

···
     "maxminddb"
   ];
 
+  # The multiprocessing tests fail on Darwin because multiprocessing uses spawn instead of fork,
+  # resulting in an exception when it can’t pickle the `lookup` local function.
+  disabledTests = lib.optionals stdenv.isDarwin [ "multiprocessing" ];
+
   meta = with lib; {
     description = "Reader for the MaxMind DB format";
     homepage = "https://github.com/maxmind/MaxMind-DB-Reader-python";