[postgres] support asyncpg as an alternative to psycopg2

[dominijk]

Make sure these boxes are checked before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Superset version

Latest

Expected results

Test enhancement with alternate database connectors

Actual results

After reading this post on async I'm interested in exploring alternate python postgres connectors, where do I start?

Steps to reproduce

[mistercrunch]

Relevant MagicStack/asyncpg#6

[stale]

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