When using the KubernetesPodOperator on an aws kubernetes cluster, the aws-iam-authenticator is used to obtain kubernetes authentication tokens. The aws tokens contain ISO-8601 formatted timestamps, which couldn't be parsed in case of a "Z" (Zulu Time) timezone. This PR fixes this problem by converting the "Z" timezone into a regular "+0000" format.
Upon further review this is only a problem with python version <= 3.6. But that should not keep the issue from being fixed.