bug: restore support for filled contour interior polygons

[wpbonelli]

FloPy 3.4.3+ drops interior polygons from filled contour geometries exported by export_contourf() (see #1951). Though it seems unlikely that many users rely on this feature, this is a breaking change.

It may worth reimplementing contour export, e.g. with contourpy, to avoid the current gymnastics to handle different MPL versions and restore support for interior polygons.