sqlalchemy pytest fixtures

The benefits for these are they can:

* makes writing sql tests easy, fast, and robust.
* Run in parallel, in separate schemas per connection.
* Only create the tables once for each pytest session.

Example:

>>> def test_bla(session):
        from bla.models import Bla
        bla = Bla(name="hell")
        assert bla.name == 'hell'
        session.add(bla)
        session.commit()

Here you can see the nesting of transactions and db code as it happens.

Creates an engine per pytest.session. Creates a connection per pytest.session. transaction create unique schema for these tests create tables nested transaction sqlalchemy.session user test code.