Fix expand-role to use boto_session field

[winstonaws]

Description of changes:

sagemaker.Session's expand_role method needs to use its boto_session field to call IAM, so it can use the configuration passed in by the user.

Merge Checklist

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[codecov-io]

Codecov Report

Merging #232 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #232   +/-   ##
=======================================
  Coverage   91.47%   91.47%           
=======================================
  Files          45       45           
  Lines        3166     3166           
=======================================
  Hits         2896     2896           
  Misses        270      270

Impacted Files	Coverage Δ
src/sagemaker/session.py	86.7% <0%> (ø)	⬆️

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