sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

as a scalar subquery such as within an IN would receive inappropriate
substitutions from the enclosing query, if the same table were present
inside the subquery as were in the enclosing query such as in a
joined inheritance scenario.
fixes #3130

to get all flake8 passing

UNION would wrap the union in an anonymous (e.g. unlabled) subquery.
fixes #3044

- implement for SQL server, use window functions when simple limit/offset not available

Removed ungrammatical apostrophes from documentation, replacing
"it's" with "its" where appropriate (but in a few cases with "it is"
when that read better).

While doing that, I also fixed a couple of minor typos etc.
as I noticed them.

Found using: https://github.com/intgr/topy