support of shared-credentials-file

HI, why shared-credentials-file is not valid method?
`            if (
                    not os.environ.get('AWS_ACCOUNT_ID', None) and
                    not (boto3.Session().get_credentials().method in ['iam-role', 'assume-role', 'assume-role-with-web-identity'])
            ):
                raise EnvironmentError('Environment variable `AWS_ACCOUNT_ID` not set and no role found.')
`