ci: isort: only target spond and tests dirs, as cannot get reliab…

…le behaviour which matches equivalent local command in root directory

.github/workflows/python-package.yml

@@ -34,10 +34,14 @@ jobs:
     - name: Lint imports with isort
       # Use command line due to bugs/doc gaps with official `isort/isort-action`.
       # Exit with error if the code is not properly formatted; show diffs;
-      # `black` compatibility
+      # `black` compatibility.
+      # The diffs should be the same as fixes made by running `isort .` in the root
+      # project folder, which picks up config from `pyproject.toml` and checks
+      # formatting in these directories
       run: |
         source $VENV
-        isort . --check-only --diff --profile black
+        isort spond --check-only --diff --profile black
+        isort tests --check-only --diff --profile black
     - name: Lint with black
       # by default: exit with error if the code is not properly formatted; show diffs
       uses: psf/black@stable