Suppose you have a model class with a primary key.

Base = declarative_base()
class User(Base):
  id = Column(BigInteger, primary_key=True)
  name = Column(String)

Previously, running
`bulk_update_mappings(User, {'id': 1, 'name': 'hello'})`
would emit the following:

```UPDATE users SET id=1, name='hello' WHERE id=1```

This is contrary to the stated behaviour, where primary keys are omitted
from the SET clause. Furthermore, this behaviour is harmful, as it
can cause the db engine to lock over-aggresively (at least in Postgres).

With this change, the emitted SQL is:

```UPDATE users SET name='hello' WHERE id=1```

objects that made use of the ``__clause_element__()`` method and
returned an object that was an ORM-mapped
:class:`.InstrumentedAttribute` and not explicitly a
:class:`.ColumnElement` would fail to be correctly
handled when passed as an expression to :meth:`.Session.query`.
The logic in 0.9 happened to succeed on this, so this use case is now
supported. fixes #3448

constructor removal; fixes #3446

object would fail to accommodate the labeled SQL expression
in all cases, such that any SQL operation that made use of
:meth:`.Label.self_group` would use the original unadapted
expression.  One effect of this would be that an ORM :func:`.aliased`
construct would not fully accommodate attributes mapped by
:obj:`.column_property`, such that the un-aliased table could
leak out when the property were used in some kinds of SQL
comparisons.
fixes #3445

in #3349

new begin, as begin_nested() does a flush

that a Python descriptor is no longer used; repairs inspectability
of these methods including support for Sphinx documentation.