pep-249 exception names linked to exception classes of an entirely
different name, preventing SQLAlchemy's own exception wrapping from
wrapping the error appropriately.
The SQLAlchemy dialect in use needs to implement a new
accessor :attr:`.DefaultDialect.dbapi_exception_translation_map`
to support this feature; this is implemented now for the py-postgresql
dialect.
fixes #3421

sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing

- build a new Sphinx extension that allows dialect info
to be entered as directives which is then rendered consistently
throughout all dialect/dbapi sections
- break out the "empty_strings" requirement for oracle test

- begin consolidating docs for dialects to be more self contained
- add a separate section for "external" dialects
- not sure how we're going to go with this yet.