Called after the :meth:`.Engine.dispose` method is called.

:func:`.engine_from_config` were not being parsed correctly;
these included ``pool_threadlocal`` and the psycopg2 argument
``use_native_unicode``. fixes #3435
- add legacy_schema_aliasing config parsing for mssql
- move use_native_unicode config arg to the psycopg2 dialect

the URL design a little simpler

and the database can no longer be connected towards, that the checkout
handler failure is caught, the attempt to re-acquire the connection
also raises an exception, but the underlying connection record
is not immediately re-checked in before the exception is propagated
outwards, having the effect that the checked-out record does not close
itself until the stack trace it's associated with is garbage collected,
preventing that record from being used for a new checkout until we
leave the scope of the stack trace.   This can lead to confusion
in the specific case of when the number of current stack traces
in memory exceeds the number of connections the pool can return,
as the pool will instead begin to raise errors about no more checkouts
available, rather than attempting a connection again.   The fix
applies a checkin of the record before re-raising.
fixes #3419

using descriptors; ensure that mock.patch() honors descriptor setters

old / new style, direct access, and ad-hoc patching and
unpatching as possible

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

passed as a keyword argument to the :meth:`.DDLEvents.before_create`,
:meth:`.DDLEvents.after_create`, :meth:`.DDLEvents.before_drop`, and
:meth:`.DDLEvents.after_drop` events would no longer be a list
of tables, but instead a list of tuples which contained a second
entry with foreign keys to be added or dropped.  As the ``tables``
collection, while documented as not necessarily stable, has come
to be relied upon, this change is considered a regression.
Additionally, in some cases for "drop", this collection would
be an iterator that would cause the operation to fail if
prematurely iterated.   The collection is now a list of table
objects in all cases and test coverage for the format of this
collection is now added.
fixes #3391

:paramref:`.Pool.reset_on_return` parameter as a synonym for ``None``,
so that string values can be used for all settings, allowing
.ini file utilities like :func:`.engine_from_config` to be usable
without issue.
fixes #3375