Await call to async method AioAssumeRoleProvider._load_creds_via_assu…

[puzza007]

Fixes #854

Description of Change

Fix async method call which wasn't awaited and results in

    frozen_credentials = await self._source_credentials.get_frozen_credentials()
AttributeError: 'coroutine' object has no attribute 'get_frozen_credentials'

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

[thehesiod]

thank you! want to add this to changes with a new dot release?

[puzza007]

Sure! Bumped version and added change to CHANGES.rst

[thehesiod]

I'm trying to see if I can find a test to catch this, if you can spot it in botocore lemme know so we can port over

[thehesiod]

found it test_recursive_assume_role, now to see the best/easiest way to port it

[puzza007]

Beat me to it - thanks!

[thehesiod]

test in #859