fix: Support for Airflow 3.1

[tomasfarias]

Closes: #166

Adds support for Airflow version 3.1 and later.

This new version of Airflow introduced a lot of changes to module import paths: Classes like DAG now come from airflow.sdk. Moreover, some XCOM methods have been removed from operator classes. They are now available only in the TaskInstance.

Moreover, and more painfully, our test suite broke with 3.1. This required a long and painful process to track which fixtures we need for things to work again.

Changes:

• When possible, import from the compatibility module, which is offered by Airflow to work across multiple Airflow versions.
• Update xcom_push method to use the TaskInstance in newer versions of Airflow.
• Add new fixtures required for Airflow 3.1 testing.
• Add Airflow 3.1 to testing pipeline.
• Drop support for Python 3.9.