sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing

fixes #3110

when that :class:`.Column` is referred to in an explicit
:class:`.PrimaryKeyConstraint` for that table.  This behavior now
matches that of when the :class:`.Column` itself has the
:paramref:`.Column.primary_key` flag set to ``True``, which is
intended to be an exactly equivalent case.
fixes #3023

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

specify a :func:`.text` expression as the target; the index no longer
needs to have a table-bound column present if the index is to be
manually added to the table, either via inline declaration or via
:meth:`.Table.append_constraint`. fixes #3028

If autoloading of a table fails, don't register it in a metadata
instance. It seems that the original behaviour was accidentally
changed in f6198d9abf453182f4b111e0579a7a4ef1614e79, restore it.

Closes issue #2988