Don't install SQLAlchemy/Pendulum adapters for other DBs (#18745)
Don't install SQLAlchemy/Pendulum adapters for other DBs (#18745)

This stops the MySQL libs being imported "unnecessarily" when Postgres
is in use -- and there have been a few confusing reports of the mysql
client libs causing problems in rare cases, so lets avoid the import if
we can.
bolkedebruin/airflow,bolkedebruin/airflow,bolkedebruin/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,apache/airflow,danielvdende/incubator-airflow,apache/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,apache/airflow,cfei18/incubator-airflow,apache/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,apache/incubator-airflow,apache/airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,lyft/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,lyft/incubator-airflow,mistercrunch/airflow,lyft/incubator-airflow,mistercrunch/airflow,apache/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow