Fix a number of ResourceWarnings when running the test suite

[falbrechtskirchinger]

Pull Request Checklist

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

Running the test suite with warnings enabled generates a large number of warnings because of open file descriptors. This PR closes the two offending file handles and also a few others found in the same code files. Two Four warnings remain, one of which is outside this project's control.

[pawamoy]

Nice!

[codecov-commenter]

Codecov Report

Patch coverage: 75.00% and project coverage change: -0.01 ⚠️

Comparison is base (3269dfd) 85.28% compared to head (ab2ec34) 85.27%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1915      +/-   ##
==========================================
- Coverage   85.28%   85.27%   -0.01%     
==========================================
  Files          99       99              
  Lines        9151     9155       +4     
  Branches     1987     1990       +3     
==========================================
+ Hits         7804     7807       +3     
- Misses        915      916       +1     
  Partials      432      432              

Flag	Coverage Δ
unittests	85.07% <75.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/pdm/installers/uninstallers.py	80.51% <50.00%> (-0.32%)	⬇️
src/pdm/installers/installers.py	80.26% <100.00%> (+0.13%)	⬆️
src/pdm/termui.py	90.47% <100.00%> (+0.06%)	⬆️

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