Fix isort's configuration

For some reason, `isort` treats a standalone import `from airflow` as a
first party import. Since `airflow` is a third party library, `isort`
has to be told to put these imports into the appropriate section.

setup.cfg

@@ -58,6 +58,7 @@ profile = black
 from_first = True
 line_length = 79
 known_first_party = egon, egon.data
+known_third_party = airflow
 default_section = THIRDPARTY
 skip = .tox,.eggs,ci/bootstrap.py,ci/templates,build,dist