vault_aws_auth_python3

#!/usr/bin/env python

import boto3
import json
import base64

def headers_to_go_style(headers):
    retval = {}
    for k, v in headers.items():
        retval[k] = [v]
    return retval

def generate_vault_request(role_name=""):
    session = boto3.session.Session()
    # if you have credentials from non-default sources, call
    # session.set_credentials here, before calling session.create_client
    client = session.client('sts')
    endpoint = client._endpoint
    operation_model = client._service_model.operation_model('GetCallerIdentity')
    request_dict = client._convert_to_request_dict({}, operation_model)

    awsIamServerId = 'vault.example.com'
    request_dict['headers']['X-Vault-awsiam-Server-Id'] = awsIamServerId

    request = endpoint.create_request(request_dict, operation_model)
    # It's now signed...
    return {
        'iam_http_request_method': request.method,
        'iam_request_url':         base64.b64encode(request.url.encode('ascii')),
        'iam_request_body':        base64.b64encode(request.body.encode('ascii')),
        'iam_request_headers':     base64.b64encode(json.dumps(headers_to_go_style(dict(request.headers)))), # It's a CaseInsensitiveDict, which is not JSON-serializable        
        'role':                    role_name,
    }

if __name__ == "__main__":
    print(json.dumps(generate_vault_request('TestRole')))

Line 9: iteritems -> items (2to3)
Line 36: print -> print() (2to3)

Line 31: TypeError: Object of type 'bytes' is not JSON serializable
This works for python2 but doesn't for python3 - is this why you had written the original program in python2? Is there a way to do this in python3?
Sorry as this is no longer about Vault but I'm not a very experienced programmer ☹️

Anyone made it work with Python3? If so, how you get rid of "Line 31: TypeError: Object of type 'bytes' is not JSON serializable" error?

b64encode needs it in byte format , so you need to encode it into that format from str, but then we need it back in str format so we need to later decode it. So we can overcome this problem by changing:
'iam_request_headers': base64.b64encode(json.dumps(headers_to_go_style(dict(request.headers)))),
to:
'iam_request_headers': base64.b64encode(json.dumps(headers_to_go_style(dict(request.headers))).encode()).decode(),