to get all flake8 passing

event.listen and event.listen_for have a kwarg once added in 0.9.4 (not 0.9.3) CHANGELOG agrees with this as well. (as does my manual testing)

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.

and :func:`.event.listens_for`.  This is a convenience feature which
will wrap the given listener such that it is only invoked once.

methods, classes, builtins, functools.partial(), everything known so far
- use get_callable_argspec() within ColumnDefault._maybe_wrap_callable, re: #2979

level (e.g. on the :class:`.Mapper` or :class:`.ClassManager`
level, as opposed to on an individual mapped class, and also on
:class:`.Connection`) that also made use of internal argument conversion
(which is most within those categories) would fail to be removable.
fixes #2973

is done

concurrent ability to return connections from the pool means that the
"first_connect" event is now no longer synchronized either, thus leading
to dialect mis-configurations under even minimal concurrency situations.