Fix a minor and inconsequential inconsistency in `copyable_holder_cas…

…ter`: the correct `load_value()` return type is `void` (as defined in `type_caster_generic`)

For easy future reference, this is the long-standing inconsistency:

* https://github.com/pybind/pybind11/blob/dbf848aff7c37ef8798bc9459a86193e28b1032f/include/pybind11/detail/type_caster_base.h#L634

* https://github.com/pybind/pybind11/blob/dbf848aff7c37ef8798bc9459a86193e28b1032f/include/pybind11/cast.h#L797

Noticed in passing while working on PR #5213.

include/pybind11/cast.h

@@ -794,11 +794,11 @@ struct copyable_holder_caster : public type_caster_base<type> {
         }
     }
 
-    bool load_value(value_and_holder &&v_h) {
+    void load_value(value_and_holder &&v_h) {
         if (v_h.holder_constructed()) {
             value = v_h.value_ptr();
             holder = v_h.template holder<holder_type>();
-            return true;
+            return;
         }
         throw cast_error("Unable to cast from non-held to held instance (T& to Holder<T>) "
 #if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)