Commenting out const in def_buffer(... const). With this, test_buffers builds...

Commenting out const in def_buffer(... const). With this, test_buffers builds and runs with PYBIND11_USE_SMART_HOLDER_AS_DEFAULT. Explanation why the const needs to be removed, or fix elsewhere, is still needed, but left for later.

include/pybind11/pybind11.h

@@ -1409,7 +1409,8 @@ public:
 
     template <typename Return, typename Class, typename... Args>
     class_ &def_buffer(Return (Class::*func)(Args...) const) {
-        return def_buffer([func] (const type &obj) { return (obj.*func)(); });
+        // NEEDED: Explanation why the const needs to be removed, or fix elsewhere.
+        return def_buffer([func] (/*const*/ type &obj) { return (obj.*func)(); });
     }
 
     template <typename C, typename D, typename... Extra>