feat: respect .python-version file

[frostming]

Signed-off-by: Frost Ming me@frostming.com

Pull Request Checklist

• 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]

Codecov Report

Attention: Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.21%. Comparing base (1d44e71) to head (0229cd4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pdm/project/core.py	81.81%	1 Missing and 1 partial ⚠️
src/pdm/cli/commands/use.py	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3367      +/-   ##
==========================================
+ Coverage   84.71%   85.21%   +0.49%     
==========================================
  Files         112      112              
  Lines       11467    11482      +15     
  Branches     2510     2515       +5     
==========================================
+ Hits         9714     9784      +70     
+ Misses       1218     1171      -47     
+ Partials      535      527       -8     

Flag	Coverage Δ
unittests	85.01% <82.35%> (+0.41%)	⬆️

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