Leverage aiohttp internal handling of chunked argument

[jakob-keller]

Description of Change

#1077 (review)

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 86.12%. Comparing base (ec33cb7) to head (086154d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1096      +/-   ##
==========================================
- Coverage   86.28%   86.12%   -0.17%     
==========================================
  Files          62       62              
  Lines        5913     5908       -5     
==========================================
- Hits         5102     5088      -14     
- Misses        811      820       +9     

Flag	Coverage Δ
unittests	86.12% <100.00%> (-0.17%)	⬇️

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