Require SQLAlchemy 1.4 (#22114)
Require SQLAlchemy 1.4 (#22114)

Some of the exceptions have moved between 1.3 and 1.4
(`sqlalchemy.orm.exc import MultipleResultsFound` becomes `from
sqlalchemy.exc import MultipleResultsFound`)

Since we only ever test with the "latest" version we should update the
requirement to be tighter to ease upgrades.

(Without this the UI/API will fail to start)
danielvdende/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,lyft/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,apache/airflow,nathanielvarona/airflow,apache/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow