It is not possible to use the What3wordswhat3words API without an API key.
The what3words API is governed by specific terms of use outlined in their API Licence Agreement: https://what3words.com/api-licence-agreement
One can sign up for a free account at https://accounts.what3words.com/select-plan?referrer=/public-api. This gives access to their core API with a generous free tier (e.g. ~1,000–10,000 requests/month depending on use case).
With a Free account, it is limited to 10 requests per second, so create a function that delays the request utilizing the time.sleep(1 / 10) Python package.