Adopt upstream unit tests session.TestCreateClient

[jakob-keller]

Description of Change

• Adopt upstream unit tests for session:
  • TestSessionPartitionFiles
  • TestGetServiceModel
  • TestCreateClient
  • TestClientMonitoring

Assumptions

Replace this text with any assumptions made (if any)

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.38%. Comparing base (7009ebe) to head (b49b6a9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1309      +/-   ##
==========================================
+ Coverage   89.81%   90.38%   +0.57%     
==========================================
  Files          67       68       +1     
  Lines        6055     6336     +281     
==========================================
+ Hits         5438     5727     +289     
+ Misses        617      609       -8     

Flag	Coverage Δ
unittests	90.38% <100.00%> (+0.57%)	⬆️

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