Fixed source profile impl

[victorskl]

• Should fetch credentials with assume-role when role-arn avail. And just need to check source profile has valid SSO login session.
• Follow how aws cli done with source profile and sts:AssumeRole explain here: https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-role.html

[codecov[bot]]

Codecov Report

Merging #14 into master will decrease coverage by 2.89%. The diff coverage is 84.31%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   97.38%   94.49%   -2.90%     
==========================================
  Files           1        1              
  Lines         191      236      +45     
==========================================
+ Hits          186      223      +37     
- Misses          5       13       +8     

Flag	Coverage Δ
#unittests	94.49% <84.31%> (-2.90%)	:arrow_down:

Impacted Files	Coverage Δ
yawsso/cli.py	94.49% <84.31%> (-2.90%)	:arrow_down:

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