The :meth:`.Connection.connect` and :meth:`.Connection.contextual_connect`

The :meth:`.Connection.connect` and :meth:`.Connection.contextual_connect`
methods now return a "branched" version so that the :meth:`.Connection.close`
method can be called on the returned connection without affecting the
original.   Allows symmetry when using :class:`.Engine` and
:class:`.Connection` objects as context managers.