Fix unsiged S3 configuration when using in deployment

[munrojm]

Make sure the boto session is not unsigned when using the client in an AWS deployment.

[codecov-commenter]

Codecov Report

Merging #583 (be81cab) into main (683ec9d) will increase coverage by 0.06%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #583      +/-   ##
==========================================
+ Coverage   92.11%   92.17%   +0.06%     
==========================================
  Files          34       34              
  Lines        1382     1380       -2     
==========================================
- Hits         1273     1272       -1     
+ Misses        109      108       -1     

Impacted Files	Coverage Δ
src/mp_api/routes/charge_density.py	86.56% <50.00%> (+1.05%)	⬆️

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