_utils: Make sure boto3's default session is used

[mrshu]

Description of changes:

• Currently awswrangler ensures that at least some boto3 Session is
  used when it calls AWS resources. However, since it creates a
  boto3.Session() when non-null boto3 Session is passed as a
  parameter, it completely sidesteps the ability of the user to set
  parameters to the boto3 session that gets used by default (via the
  boto3.setup_default_session function).

• This commit ensures that the ensure_session function uses boto3's
  default session (reachable via boto3._get_default_session()) which
  then also allows awswrangler users to set default parameters of the
  boto3 Session it uses by default via boto3.setup_default_session().

Signed-off-by: mr.Shu mr@shu.io

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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Relevant link: https://boto3.amazonaws.com/v1/documentation/api/latest/_modules/boto3.html#setup_default_session

[mrshu]

@igorborgest I am not sure how would one test something like this but I'd be happy to incorporate some tests, if that was advisable.

Furthermore, if this ever makes it to master it may make sense to update the second part of the tutorial, so that users are aware that the parameters of the default session can also be set globally -- much like in the pre-1.0.0 versions of awswrangler.

Please do let me know if there is anything you'd like me to update here.

Thanks!

[igorborgest]

Hi @mrshu, thanks for the contribution.

It makes a lot of sense.

I would like to avoid access to external private methods as much as possible, but I can't see an alternative by now.

I will merge it on the dev branch and then write some tests to cover it.