Standalone pre-commit hook

[francescorubbo]

The pre-commit hook shouldn't depend on the repo environment (see this SO post by the pre-commit creator). This requires a standalone hook configuration that defines the dependency on a specific version of deptry. I recently created such standalone hook for my own convenience (I plan to keep it in sync with deptry releases):
https://github.com/francescorubbo/deptry-pre-commit

Perhaps it can be useful for amending the deptry doc to align with the recommended usage of pre-commit.