Aws-export-credentials

* Update config_file_writer.py for boto3 (17)

> ⚠️ This version has a bug in `--credentials-file-profile` (17), use [v0.18](https://github.com/benkehoe/aws-export-credentials/releases/tag/v0.18) instead.

* Added `--ensure-temporary` to turn static credentials for IAM Users and account root into temporary creds with [STS.GetSessionToken](https://docs.aws.amazon.com/STS/latest/APIReference/API_GetSessionToken.html).
* Always use temporary credentials for `--imds` and `--container`.

* Use subprocess module for `--exec`
* Remove support for Python 3.5 and 3.6

* Update expiration field names to match AWS names (12)

* Fix serialization of datetime object to JSON (10)
* Handle IAM Users for IMDS