Are AWS Boto3 calls asynchronous? In the API documentation for Apache Cloud Stack for example, they put an (A) to designate asynchronous API calls.

In the Boto3 - Python client API for AWS - documentation as I can't see anything like this - so my question is whether its API is asynchronous or not.
Boto3 is a client for AWS API, so by definition it doesn't handle the synchronous or asynchronous behavior of the API call, that's the API endpoint which define that.

You can safely assume that any API endpoint marked as asynchronous will be asynchronous in any API client. Some SDK implement methods to wait for those asynchronous calls, some don't and you need to write your own handling (usually polling regularly the state) if you need to wait the end of a task started by an API call (creating a CloudFormation stack for exemple).