Limit AsyncAdaptedQueue to Python 3.7

Limit AsyncAdaptedQueue to Python 3.7

Tests here are failing for python 3.6 due to the lack
of asyncio.run().   It seems to be non-trivial to vendor
a working version of this in Python 3.6 as the tests here
are running it in alternate threads.

The python documentation imports everything directly from the
asyncio package, and it seems that py < 3.8 does not have the
asyncio.exception module

Closes: #5865
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5865
Pull-request-sha: 35cc1fa3f6ff962676f571ae30851f4b4d96762a

Change-Id: I9398c9fb2aa87f3228ce2f59277de732091bd541