Pending deprecation of layout methods

The methods set_tight_layout, set_constrained_layout, are discouraged, and now emit a PendingDeprecationWarning in favor of explicitly referencing the layout engine via figure.set_layout_engine('tight') and figure.set_layout_engine('constrained'). End users should not see the warning, but library authors should adjust.

The methods set_constrained_layout_pads and get_constrained_layout_pads are will be deprecated in favor of figure.get_layout_engine().set() and figure.get_layout_engine().get(), and currently emit a PendingDeprecationWarning.