Fix get_hinting_flag for Matplotlib 3.10.

CHANGELOG.rst

@@ -2,6 +2,7 @@ next
 ====
 
 - Bumped dependencies to Python≥3.8.
+- Fixed compatibility with Matplotlib≥3.10.
 - Fixed support for macos backend on Matplotlib≥3.6.
 - Fixed compatibility with wheel≥0.41.
 - Support pdf MaxVersion up to 1.7 (if the underlying cairo supports it).

ext/_util.cpp

@@ -862,9 +862,11 @@ std::vector<cairo_font_face_t*> font_faces_from_prop(py::object prop)
 long get_hinting_flag()
 {
   // FIXME[matplotlib]: Should be moved out of backend_agg.
-  return
+  auto const& hf =
     py::module::import("matplotlib.backends.backend_agg")
-    .attr("get_hinting_flag")().cast<long>();
+    .attr("get_hinting_flag")();
+  // Matplotlib 3.10 changed the flag from int to enum.Flag.
+  return (py::hasattr(hf, "value") ? hf.attr("value") : hf).cast<long>();
 }
 
 void adjust_font_options(cairo_t* cr, bool subpixel_antialiased_text_allowed)