Dependency updates

[bensteinberg]

In order to update urllib3, I had to update boto3 and botocore, as well as s3transfer:

https://urllib3.readthedocs.io/en/latest/changelog.html

https://github.com/boto/boto3/blob/develop/CHANGELOG.rst

https://github.com/boto/botocore/blob/develop/CHANGELOG.rst

Although the s3transfer README says

If you are planning to use this code in production, make sure to lock to a minor version as interfaces may break from minor version to minor version.

the changelog looks ok:

https://github.com/boto/s3transfer/blob/develop/CHANGELOG.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.84%. Comparing base (b70124e) to head (bde0aa3).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3598   +/-   ##
========================================
  Coverage    68.84%   68.84%           
========================================
  Files           48       48           
  Lines         7004     7004           
========================================
  Hits          4822     4822           
  Misses        2182     2182           

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[rebeccacremona]

LGTM!