Fix leak in var arg handling

When using the mixed position + vararg path, pybind over inc_ref's
the vararg positions. Printing the ref_count() of `item` before
and after this change you see:

Before change:

```
refcount of item before assign 3
refcount of item after assign 5
```

After change
```
refcount of item before assign 3
refcount of item after assign 4
```

include/pybind11/pybind11.h

@@ -581,7 +581,7 @@ protected:
                         extra_args = tuple(args_size);
                         for (size_t i = 0; i < args_size; ++i) {
                             handle item = PyTuple_GET_ITEM(args_in, args_copied + i);
-                            extra_args[i] = item.inc_ref().ptr();
+                            extra_args[i] = reinterpret_borrow<object>(item);
                         }
                     }
                     call.args.push_back(extra_args);