Add support for PEP 621

[mkniewallner]

Resolves #109.

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

Add support for PEP 621 dependency specification. This will ensure that modern package managers that follow PEP 621 are supported.

The implementation makes the assumption that optional dependencies under [project.optional-dependencies] are all non-development ones. In a follow up PR, #162 (comment) could be an interesting option to have to define groups that should be considered development dependencies groups.

Since PDM uses PEP 621 (with an additional logic for development dependencies), this PR also updates its dependency getter class so that it subclasses the PEP 621 one in order to avoid duplication.

The dependency specification detector is also slightly updated so that projects using PDM will only use PDM dependency getter class if they define development dependencies under [tool.pdm.dev-dependencies]. Otherwise, we fallback to PEP 621 dependency getter class, since there is no PDM specifics related to dependencies.

[fpgmaas]

I think we could remove the 'Draft' mark from this PR, looks good to me. We can see how we continue with the development dependencies in a new issue.