python3Packages.testpath: fix tests on Darwin

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

@@ -1,4 +1,5 @@
 { lib
+, stdenv
 , buildPythonPackage
 , fetchPypi
 , pytestCheckHook
@@ -17,6 +18,11 @@ buildPythonPackage rec {
     pytestCheckHook
   ];
 
+  preCheck = lib.optionalString stdenv.isDarwin ''
+    # Work around https://github.com/jupyter/testpath/issues/24
+    export TMPDIR="/tmp"
+  '';
+
   meta = with lib; {
     description = "Test utilities for code working with files and commands";
     license = licenses.mit;