docs(changelog): add 0.21.0 release notes

[mkniewallner]

Provisional release notes for 0.21.0.

Things I'd like to see included first (but not blockers):

• Understand more complex requirements.txt files, follow into nested/included files #912
• .gitignore from parent git repository is used while it shouldn't #882
• Imports with package names different from module names are wrongly reported as missing instead of transitive #827
• feat(pep621): use packaging-based PEP 508 parser #925

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.0%. Comparing base (975ee16) to head (8352f39).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #907   +/-   ##
=====================================
  Coverage   93.0%   93.0%           
=====================================
  Files         37      37           
  Lines        991     991           
  Branches      97      97           
=====================================
  Hits         922     922           
  Misses        55      55           
  Partials      14      14           

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[mkniewallner]

Will probably defer #827 to the next release, as it's kinda related to #861, which I'm still hesitant on, and we already have several changes and new features in this release.

I'd still like to see #925 in the release, but not a blocker neither.