refactor: Make apache-airflow not required

Since we wish to support all MWAA supported versions of Airflow,
this requires supporting both 1.10.12 and 2.X releases. However, There
are several conflicting dependencies between dbt-core and
apache-airflow. None of the conflicts should break us, but they do
make dependency resolution impossible, which means airflow-dbt-python
cannot be installed.

For this reason we have removed the dependency to
apache-airflow. Assuming that as an Airflow operator we are always
installing airflow-dbt-python in an environment that already has
Airflow installed, this shouldn't cause any issues in production
deployments.

Moreover, this makes testing multiple Airflow versions easier as we
can pre-install apache-airflow.

Author: tomasfarias

.github/workflows/test.yaml

@@ -26,7 +26,7 @@ jobs:
         with:
           poetry-version: 1.1.12
       - name: Install airflow-dbt-python with Poetry
-        run: poetry install -E amazon -E postgres
+        run: poetry install -E amazon -E postgres -E airflow
       - name: Style guide enforcement with flake8
         run: poetry run flake8 .
       - name: Static type checking with mypy