Use rvalue reference for std::variant cast_op<T> (#3811)

Nearly every call site of cast_op<T> uses an r-value reference.

Except stl.h variant_caster::load_alternative for handling std::variant.

Fix that.

Author: laramiel

include/pybind11/stl.h

@@ -372,7 +372,7 @@ struct variant_caster<V<Ts...>> {
     bool load_alternative(handle src, bool convert, type_list<U, Us...>) {
         auto caster = make_caster<U>();
         if (caster.load(src, convert)) {
-            value = cast_op<U>(caster);
+            value = cast_op<U>(std::move(caster));
             return true;
         }
         return load_alternative(src, convert, type_list<Us...>{});