Tighten file permissions when writing credentials in CodeArtifac…

[FiveSlashNine]

Description of changes:
This change updates the CodeArtifact login credential file operations to use secure file permissions (0o600) by replacing standard open() calls with os.open() and os.fdopen(). This prevents credentials in .netrc and .pypirc files from being created with default permissions that could expose them to unauthorized users on the system.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.42%. Comparing base (c4681b5) to head (43c68a7).
⚠️ Report is 251 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #10191      +/-   ##
===========================================
+ Coverage    93.39%   93.42%   +0.03%     
===========================================
  Files          210      210              
  Lines        17052    17227     +175     
===========================================
+ Hits         15925    16095     +170     
- Misses        1127     1132       +5     

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[AndrewAsseily]

Thanks!