Add doctype

[q0w]

Pull Request Check List

Its a temporary solution, should migrate to html.parser as html5lib will be deprecated. New parser requires doctypes.
Or add html5lib/another parser as a dependency.

• 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.

[codecov-commenter]

Codecov Report

Merging #887 (5672d85) into main (42edab5) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #887      +/-   ##
==========================================
+ Coverage   83.75%   83.77%   +0.01%     
==========================================
  Files          74       74              
  Lines        6359     6359              
  Branches     1316     1316              
==========================================
+ Hits         5326     5327       +1     
+ Misses        711      710       -1     
  Partials      322      322              

Flag	Coverage Δ
unittests	83.53% <ø> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
pdm/models/pip_shims.py	88.67% <0.00%> (+1.88%)	⬆️

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