Passing AWS role to the application that uses default boto3 configs

I have an aws setup that requires me to assume role and get corresponding credentials in order to write to s3. For example, to write with aws cli, I need to use --profile readwrite flag. If I write code myself with boot, I'd assume role via sts, get credentials, and create new session.

However, there is a bunch of applications and packages relying on boto3's configuration, e.g. internal code runs like this:

s3 = boto3.resource('s3')
result_s3 = s3.Object(bucket, s3_object_key)
result_s3.put(
                Body=value.encode(content_encoding),
                ContentEncoding=content_encoding,
                ContentType=content_type,
        )

From documentation, boto3 can be set to use default profile using (among others) AWS_PROFILE env variable, and it clearly "works" in terms that boto3.Session().profile_name does match the variable - but the applications still won't write to s3.

What would be the cleanest/correct way to set them properly? I tried to pull credentials from sts, and write them as AWS_SECRET_TOKEN etc, but that didn't work for me...