You can use the "suds" python library to call the SOAP API from Odoo.

Ex:

from suds.client import Client

soap_msg = '''<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
..... some soap message
</soap:Envelope>'''

client = Client('https://soap_url_of_WSDL?WSDL')
message = client.factory.create('Message')
clientheader = client.factory.create('ClientHeader')
# Add required details in HEADER as per WSDL rule
clientheader.user = 'test' # credentials / token keys
clientheader.SendAttempt = '0'
message.ClientHeader = clientheader
res_resp = client.service.ProcessMsg(message)

I hope this will help you to give it a start.