MNT : tweak how internals of Axes.plot work

The actual processing and artist creation in Axes.plot happens through
the `__call__` method on a _process_plot_var_args instance hanging off
the Axes object.  The _process_plot_var_args knows what axes it is
hanging off of and passed that into the Line2D artists as a kwarg at
creation time, which sets the axes of the Line2D.  The axes of the Line2D
is set again during the `Axes.add_line` method.  For the most part this
is fine, if un-needed, because the axes is the same objects both time
it is set.  However when using the mpl_toolkit, Axes objects get wrapped
in not-strictly-subclassed objects and the Axes object that the
_process_plot_var_args object knows about is not the Axes object that
add_line is called on which now results in an exception as it is moving
artists between axes.