Add the option to check for obsolete dev-dependencies

[fpgmaas]

Currently, only regular dependencies are checked. dev-dependencies could also be checked, but they are often packages like jupyterlab, black, isort, flake8, which are not imported in the code. So the logic becomes a lot more fuzzy, should we e.g. check makefiles & ci/cd files for the mentioning of these dependencies?

Alternatively, those can be added to an argument dev-dependencies-to-ignore in pyproject.toml (or something similar), and regular python files can be scanned for imports. This can still help to find obsolete dependencies; e.g. a package might have been used in the development of unit tests, but it's no longer actually used.

[fpgmaas]

For now added the wontfix label, since this will be quite a large overhaul, and I am wondering if user configuration won't become too confusing. Users would need to specify separately which files and directories to include for both the regular and the development dependency checks.

On the other hand, most of the required classes and functions are already there. Optionally we could add a separate command, so users could run:

deptry .

to check regular dependencies and

deptry dev .

To check the development dependencies.

Anyway, if this issue gains attention in the future, we could always reconsider.