There is no fixed rule, where to place this python file as long as you can import somewhere inside your module. If the sole purpose of a file is to act as a driver/handler to something outside the Odoo, I usually place files like these in a handler folder inside my module (see e.g., the hw_drivers module from Odoo). If your file needs to save some variables within the Odoo scope, I usually save it as a model and place the class functionality there (see e.g., the payment modules from Odoo). But it really depends on your specific use case. I hope this helps!