The :meth:`.AssociationProxy.any`, :meth:`.AssociationProxy.has`
and :meth:`.AssociationProxy.contains`
comparison methods now support linkage to an attribute that
is itself also an :class:`.AssociationProxy`, recursively.

After some initial attempts it's clear that the any() / has()
of AssociationProxy needed to be reworked into a generic
_criterion_exists() to allow this to work recursively without
excess complexity.  For the case of the multi-linked associationproxy,
the usual checks of "any()" / "has()" correctness simply don't
take place; for a single-link association proxy the error
checking logic that takes place in relationship() has been
ported to the local any() / has() methods.

Change-Id: Ic5aed2a4e910b8138a737d215430113c31cce856
Fixes: #3769

Improved the association proxy list collection so that premature
autoflush against a newly created association object can be prevented
in the case where ``list.append()`` is being used, and a lazy load
would be invoked when the association proxy accesses the endpoint
collection.  The endpoint collection is now accessed first before
the creator is invoked to produce the association object.

Change-Id: I008a6dbdfe5b1c0dfd02189c3d954d83a65f3fc5
Fixes: #3941

Corrects some warnings and adds tox config.  Adds DeprecationWarning
to the error category.   Large sweep for string literals w/ backslashes
as this is common in docstrings

Co-authored-by: Andrii Soldatenko
Fixes: #3886
Change-Id: Ia7c838dfbbe70b262622ed0803d581edc736e085
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/337

Change-Id: I4e8c2aa8fe817bb2af8707410fa0201f938781de

From [PEP 479](https://www.python.org/dev/peps/pep-0479/) the correct way to
terminate a generator is to return (which implicitly raises StopIteration)
rather than raise StopIteration.

Without this change using sqlalchemy in python 3.5 or greater results in
these warnings
    PendingDeprecationWarning: generator '__iter__' raised StopIteration
which this commit should remove.

:class:`.AssociationProxy` constructor, to suit the
:attr:`.AssociationProxy.info` accessor that was added in
:ticket:`2971`.  This is possible because :class:`.AssociationProxy`
is constructed explicitly, unlike a hybrid which is constructed
implicitly via the decorator syntax.
fixes #3551

on an relationship->scalar non-object attribute comparison would fail,
e.g.
``filter(Parent.some_collection_to_attribute.any(Child.attr == 'foo'))``
fixes #3397