Support asyncpg with SQLAlchemy 1.4

[amotl]

Hi there,

within #391, I am trying to add support for SQLAlchemy 1.4. By working on it and more specifically through [1,2], I learned that

SQLAlchemy version 1.4 and later supports the asyncpg dialect natively.

So, this might be an interesting option to consider.

With kind regards, Andreas.

[1] https://docs.sqlalchemy.org/en/14/orm/extensions/asyncio.html [2] https://github.com/MagicStack/asyncpg/commit/3d0e23fe

P.S.: Also linking to https://github.com/MagicStack/asyncpg/issues/6 and https://github.com/apache/incubator-superset/issues/7018 here.

[amotl]

Appropriate SQLAlchemy support for Python's Stackless spin-off Greenlet also just got merged, see https://github.com/sqlalchemy/sqlalchemy/issues/5615 and https://github.com/sqlalchemy/sqlalchemy/pull/5616.