Remove uneeded import from code example

This had me reread the code twice to see if I missed why the import was
present.
- Support is improved for supplying a :func:`.join` construct as the
target of :paramref:`.relationship.secondary` for the purposes
of creating very complex :func:`.relationship` join conditions.
The change includes adjustments to query joining, joined eager loading
to not render a SELECT subquery, changes to lazy loading such that
the "secondary" target is properly included in the SELECT, and
changes to declarative to better support specification of a
join() object with classes as targets.
used the wrong join
Merge pull request #60 from wichert/mutable-doc-import

Remove uneeded import from code example
- Fixed an 0.9 regression where the automatic aliasing applied by
:class:`.Query` and in other situations where selects or joins
were aliased (such as joined table inheritance) could fail if a
user-defined :class:`.Column` subclass were used in the expression.
In this case, the subclass would fail to propagate ORM-specific
"annotations" along needed by the adaptation.  The "expression
annotations" system has been corrected to account for this case.
[ticket:2918]
- remove this leftover commented pdb
typo