Airflow installs WTForms 2.3.0 instead of 2.2.1, which is incompatible
Related to https://github.com/apache/airflow/issues/8506
https://github.com/apache/airflow/blob/master/requirements/requirements-python3.6.txt requires 2.2.1, but this is not actually respected by pypi's depencency resolution (see https://pip.pypa.io/en/stable/user_guide/#id14).
https://github.com/apache/airflow/blob/master/setup.py didn't require wtforms at all in 1.10.2 (required by Meltano), but requires <2.3.0
as of https://github.com/apache/airflow/pull/8512.
That fix is currently unreleased, but will be in 1.10.11.