Encode the requirments.txt from ireq

[frostming]

Pull Request Check List

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

Close #1036

[codecov-commenter]

Codecov Report

Merging #1037 (03b503e) into main (20f737b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1037      +/-   ##
==========================================
+ Coverage   83.96%   83.99%   +0.02%     
==========================================
  Files          73       73              
  Lines        6469     6468       -1     
  Branches     1344     1344              
==========================================
+ Hits         5432     5433       +1     
+ Misses        710      709       -1     
+ Partials      327      326       -1     

Flag	Coverage Δ
unittests	83.75% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
pdm/formats/requirements.py	93.47% <100.00%> (+2.08%)	⬆️
pdm/models/requirements.py	91.36% <100.00%> (ø)

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