python3Packages.jupytext: fix tests

Due to improper sandboxing, I didn't notice the failed tests while
upgrading jupytext in NixOS#121648.  The errors come from jupyter_core so
the same fix is used here as there: set $HOME to a writable directory.

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

@@ -44,7 +44,9 @@ buildPythonPackage rec {
     jupyter_client
     notebook
   ];
-  # pre-commit tests require a Git repository.
+  # Tests that use a Jupyter notebook require $HOME to be writable.
+  HOME = "$TMPDIR";
+  # Pre-commit tests expect the source directory to be a Git repository.
   pytestFlagsArray = [ "--ignore-glob='tests/test_pre_commit_*.py'" ];
   pythonImportsCheck = [ "jupytext" "jupytext.cli" ];