: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

with :meth:`.Connection.execution_options` when a :class:`.Transaction`
is in play; DBAPIs and/or SQLAlchemy dialects such as psycopg2,
MySQLdb may implicitly rollback or commit the transaction, or
not change the setting til next transaction, so this is never safe.
- Added new parameter :paramref:`.Session.connection.execution_options`
which may be used to set up execution options on a :class:`.Connection`
when it is first checked out, before the transaction has begun.
This is used to set up options such as isolation level on the
connection before the transaction starts.
- added new documentation section
detailing best practices for setting transaction isolation with
sessions.
fixes #3296

levels; :meth:`.Connection.get_isolation_level`,
:attr:`.Connection.default_isolation_level`.
- enhance documentation inter-linkage between new accessors,
existing isolation_level parameters, as well as in
the dialect-level methods which should be fully covered
by Engine/Connection level APIs now.

for psycopg2 and others, encourage users to take advantage of positional
styles by documenting "paramstyle".   A section is added to psycopg2
specifically as this is a pretty common spot for named parameters
that may be unusually named.  fixes #3246.

to get all flake8 passing

in all cases unconditionally, the number of use cases that go beyond what
dbapi_error() is expecting has gone too far for an 0.9 release.
Additionally, the number of things we'd like to track is really a lot
more than the five arguments here, and ExecutionContext is really not
suitable as totally public API for this.   So restore dbapi_error
to its old version, deprecate, and build out handle_error instead.
This is a lot more extensible and doesn't get in the way of anything
compatibility-wise.