Fix physx test path and add pint dependency to pre-commit

- test_units.py: Use absolute path based on script location
- .pre-commit-config.yaml: Add pint to additional_dependencies

.pre-commit-config.yaml

@@ -239,6 +239,7 @@ repos:
         name: "Book: PhysX unit checks"
         entry: python book/quarto/physx/test_units.py
         language: python
+        additional_dependencies: [pint]
         pass_filenames: false
         files: ^book/quarto/contents/.*\.qmd$