Deprecate usage of Python 2.7 interpreter

[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.

[codecov-commenter]

Codecov Report

Merging #1082 (eb574f9) into main (9f36e53) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main    #1082      +/-   ##
==========================================
- Coverage   83.86%   83.85%   -0.02%     
==========================================
  Files          73       74       +1     
  Lines        6525     6545      +20     
  Branches     1532     1536       +4     
==========================================
+ Hits         5472     5488      +16     
- Misses        725      727       +2     
- Partials      328      330       +2     

Flag	Coverage Δ
unittests	83.59% <70.83%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
pdm/project/core.py	89.65% <0.00%> (-0.48%)	⬇️
pdm/__version__.py	75.00% <75.00%> (ø)
pdm/utils.py	78.50% <87.50%> (+0.22%)	⬆️
pdm/core.py	88.59% <100.00%> (+1.52%)	⬆️

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