Support DNS-less connections for psycopg2
Added support for parameter-less connection URLs for the psycopg2 dialect,
meaning, the URL can be passed to :func:`.create_engine` as
``"postgresql+psycopg2://"`` with no additional arguments to indicate an
empty DSN passed to libpq, which indicates to connect to "localhost" with
no username, password, or database given. Pull request courtesy Julian
Mehnle.
Fixes: #4562
Closes: #4563
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4563
Pull-request-sha:
8a05c96944a0523b06e9772cfe1832e500a43641
Change-Id: Ib6fca3c3c9eebeaf590d7f7fb0bc8cd4b6e4a55a