python3Packages.cattrs: use pytest-xdist

Upstream uses it as well and it cuts the complete build down from
~14 minutes to ~1m30s.

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

@@ -7,6 +7,7 @@
 , motor
 , msgpack
 , poetry-core
+, pytest-xdist
 , pytestCheckHook
 , pythonOlder
 , pyyaml
@@ -44,12 +45,17 @@ buildPythonPackage rec {
     immutables
     motor
     msgpack
+    pytest-xdist
     pytestCheckHook
     pyyaml
     tomlkit
     ujson
   ];
 
+  pytestFlagsArray = [
+    "--numprocesses $NIX_BUILD_CORES"
+  ];
+
   postPatch = ''
     substituteInPlace pyproject.toml \
       --replace "-l --benchmark-sort=fullname --benchmark-warmup=true --benchmark-warmup-iterations=5  --benchmark-group-by=fullname" "" \