feat: use packaging for parsing PEP 508 dependencies

[mkniewallner]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.4%. Comparing base (63ca221) to head (d63043d).

Files	Patch %	Lines
deptry/dependency_getter/pep_621.py	87.5%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@          Coverage Diff          @@
##            main     #29   +/-   ##
=====================================
  Coverage   90.4%   90.4%           
=====================================
  Files         39      39           
  Lines       1084    1039   -45     
  Branches     220     205   -15     
=====================================
- Hits         980     940   -40     
+ Misses        89      86    -3     
+ Partials      15      13    -2     

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