the URL design a little simpler

:meth:`.URL.get_dialect` method will continue to return the
ultimate :class:`.Dialect` object when a dialect plugin is used,
without the need for the caller to be aware of the
:meth:`.Dialect.get_dialect_cls` method.
reference #3379

functionality.   Added a new "soft invalidate" feature to the
connection pool at the level of the checked out connection wrapper
as well as the :class:`._ConnectionRecord`.  This works similarly
to a modern pool invalidation in that connections aren't actively
closed, but are recycled only on next checkout; this is essentially
a per-connection version of that feature.  A new event
:class:`.PoolEvents.soft_invalidate` is added to complement it.
fixes #3379

- Added new flag
:attr:`.ExceptionContext.invalidate_pool_on_disconnect`.
Allows an error handler within :meth:`.ConnectionEvents.handle_error`
to maintain a "disconnect" condition, but to handle calling invalidate
on individual connections in a specific manner within the event.

- Added new event :class:`.DialectEvents.do_connect`, which allows
interception / replacement of when the :meth:`.Dialect.connect`
hook is called to create a DBAPI connection.  Also added
dialect plugin hooks :meth:`.Dialect.get_dialect_cls` and
:meth:`.Dialect.engine_created` which allow external plugins to
add events to existing dialects using entry points.
fixes #3355

take effect in all engine connection use cases, including
when user-custom connect routines are used via the
:paramref:`.create_engine.creator` parameter, as well as when
the :class:`.Connection` encounters a connection error on
revalidation.
fixes #3266

that union() can be called, in the case of a dialect that uses
execution options inside of initialize() (e.g. oursql)

:paramref:`.create_engine.execution_options` or
:meth:`.Engine.update_execution_options` are not passed to the
special :class:`.Connection` used to initialize the dialect
within the "first connect" event; dialects will usually
perform their own queries in this phase, and none of the
current available  options should be applied here.  In
particular, the "autocommit" option was causing an attempt to
autocommit within this initial connect which would fail with
an AttributeError due to the non-standard state of the
:class:`.Connection`.
fixes #3200

to get all flake8 passing

- Fixed bug which would occur if a DBAPI exception
occurs when the engine first connects and does its initial checks,
and the exception is not a disconnect exception, yet the cursor
raises an error when we try to close it.  In this case the real
exception would be quashed as we tried to log the cursor close
exception via the connection pool and failed, as we were trying
to access the pool's logger in a way that is inappropriate
in this very specific scenario. fixes #3063