I think the docs aren't wonderful at exposing how to do this. It has been a supported feature for some time, however, and there are some details in this pull request.
So there are three different ways to do this:
Option A) Create a new session with the profile
dev = boto3.session.Session(profile_name='dev')
Option B) Change the profile of the default session in code
boto3.setup_default_session(profile_name='dev')
Option C) Change the profile of the default session with an environment variable
$ AWS_PROFILE=dev ipython
>>> import boto3
>>> s3dev = boto3.resource('s3')
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