feat(pep621): use packaging-based PEP 508 parser

[mkniewallner]

Closes #735.

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!
• If you've added or modified a feature, documentation in docs is updated

Description of changes

In #913, we've changed the way we parse dependencies from requirements by switching to requirements-parser. This partly replaces what was done in #735.

Since the dependency we added itself depends on packaging, we can leverage it to parse dependencies defined in PEP 621, without adding a new dependency.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.0%. Comparing base (0781758) to head (d996cc6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #925     +/-   ##
=======================================
+ Coverage   92.7%   93.0%   +0.2%     
=======================================
  Files         37      37             
  Lines        992     991      -1     
  Branches      98      97      -1     
=======================================
+ Hits         920     922      +2     
+ Misses        56      55      -1     
+ Partials      16      14      -2     

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[mkniewallner]

This was never valid in the first place.