Add support for PEP621 with PDM

[fpgmaas]

This MR adds support for projects that use PDM for their dependency specification, which in turn use PEP621. The reason we add support for PDM and not for PEP621 directly is that PEP621 does not specify a unified way to specify development dependencies, see also the issue #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

[codecov-commenter]

Codecov Report

Merging #155 (025498c) into main (26e55ac) will increase coverage by 0.3%.
The diff coverage is 95.1%.

@@           Coverage Diff           @@
##            main    #155     +/-   ##
=======================================
+ Coverage   94.8%   95.1%   +0.3%     
=======================================
  Files         27      28      +1     
  Lines        808     882     +74     
  Branches     127     133      +6     
=======================================
+ Hits         766     839     +73     
  Misses        28      28             
- Partials      14      15      +1     

Impacted Files	Coverage Δ
deptry/core.py	97.2% <75.0%> (+0.1%)	⬆️
deptry/dependency_getter/pdm.py	94.1% <94.1%> (ø)
deptry/dependency_specification_detector.py	100.0% <100.0%> (+5.7%)	⬆️

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[fpgmaas]

Thanks for your feedback @mkniewallner. Accepted and processed all suggestions :)