Change-Id: I9fb9947c45e34369ebb8f55c3c52a2d6f4e879cf

Fixes pypy

Change-Id: Iadfb8bc74d47a23e50ab69e9d784652355f5e622

Fixed critical regression where the :class:`_orm.Session` could fail to
"autobegin" a new transaction when a flush occurred without an existing
transaction in place, implicitly placing the :class:`_orm.Session` into
legacy autocommit mode which commit the transaction. The
:class:`_orm.Session` now has a check that will prevent this condition from
occurring, in addition to repairing the flush issue.

Additionally, scaled back part of the change made as part of :ticket:`5226`
which can run autoflush during an unexpire operation, to not actually
do this in the case of a :class:`_orm.Session` using legacy
:paramref:`_orm.Session.autocommit` mode, as this incurs a commit within
a refresh operation.

Fixes: #6233

Change-Id: Ia980e62a090e39e3e2a7fb77c95832ae784cc9a5

Fixed critical regression caused by the new feature added as part of
:ticket:`1763`, eager loaders are invoked on unexpire operations. The new
feature makes use of the "immediateload" eager loader strategy as a
substitute for a collection loading strategy, which unlike the other
"post-load" strategies was not accommodating for recursive invocations
between mutually-dependent relationships, leading to recursion overflow
errors.

Fixes: #6232
Change-Id: Ifb2286281f40d1a04c24741261d4438659b6e3dd

Add ``supports_schema = True`` to DefaultDialect and modify
requirements.py to use that attribute so third-party dialects
can explicitly indicate that they do *not* support schemas by
specifying ``supports_schema = False`` in their Dialect class.

Change-Id: Idffee82f6668a15ac7148f2a407a17de785d1fb7

Other doc cleanups and fixes as well, including more
explicitness that declarative attribute assignment only
works when a declarative base is involved; other forms
need to call add_property() directly.

Fixes: #6225
Change-Id: I123c3fb35cfa788b96bac7ab640f6de857d003d6