bug: export_contours broken with matplotlib 3.8

[wpbonelli]

Describe the bug
With matplotlib 3.8.0, contours exported by export_contours() and export_contourf() are strangely merged/connected. This is the reason CI testing is broken at the moment

To Reproduce
Install matplotlib 3.8.0 and run the test_export.py::test_export_contours() and ...contourf() tests, then load and plot the exported shapes e.g.

with Reader(filename) as r:
    shapes = r.shapes()
    for s in shapes:
        x = [i[0] for i in s.points[:]]
        y = [i[1] for i in s.points[:]]
        plt.plot(x, y)

See #1951

Screenshots
Exported with 3.8.0:

For reference, with 3.7.2 (expected results)

[wpbonelli]

mpl's ContourSet is now a Collection with a single path for each level. Paths may contain disconnected components. So it looks like this needs modification (possibly similar for contourf too)

Edit

The root issue here was that export_contours/f() made an unsafe assumption that some matplotlib behavior was part of the stable API. Calling .get_paths() on a collection of contours now returns one path per layer, possibly containing multiple components, where previously a separate path was returned for each disjoint contour component. The export functions previously expected to straightforwardly convert paths to linestrings — now, traversing the path and checking path codes is necessary to determine connectivity and split in the right locations.

Another consideration is that filled contour interior polygons (i.e. holes) can no longer be extracted from paths.

Noting that with the change of behaviour of .collections no longer returning information about the relationship between filled polygons and their holes, we will find out who has been using mpl to obtain such information rather than just render filled contours.

Given this, we will likely want to mention in the next release notes that filled contours exported with mpl >= 3.8 will no longer contain any interior (hole) polygons.

More info:

• [MNT]: Turn ContourSet into a (nearly) plain Collection matplotlib/matplotlib#25128
• Turn ContourSet into a standard Collection artist. matplotlib/matplotlib#25247