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

or tuple would raise an IndexError.   It now produces an empty
insert construct as would be the case with an empty dictionary.

of the given names would not be taken into account when generating
the INSERT statement, thus producing a mismatch versus the column
names in the given SELECT statement.  Also noted that
:meth:`.Insert.from_select` implies that Python-side insert defaults
cannot be used, since the statement has no VALUES clause. [ticket:2895]

:meth:`.Insert.from_select`.  Given a list of columns and
a selectable, renders ``INSERT INTO (table) (columns) SELECT ..``.
While this feature is highlighted as part of 0.9 it is also
backported to 0.8.3. [ticket:722]
- The :func:`.update`, :func:`.insert`, and :func:`.delete` constructs
will now interpret ORM entities as FROM clauses to be operated upon,
in the same way that select() already does.  Also in 0.8.3.

needs to be explicit here since tablestest sticks testing.db onto metadata.bind