Make change as suggested by @laramiel. This makes it much more obviou…

…s that the latest implementation of `smart_holder_from_unique_ptr()` accepts all existing `return_value_policy` enum values except `copy`.

include/pybind11/detail/type_caster_base.h

@@ -528,13 +528,8 @@ handle smart_holder_from_unique_ptr(std::unique_ptr<T, D> &&src,
                                     return_value_policy policy,
                                     handle parent,
                                     const std::pair<const void *, const type_info *> &st) {
-    if (policy != return_value_policy::automatic
-        && policy != return_value_policy::automatic_reference
-        && policy != return_value_policy::take_ownership && policy != return_value_policy::move
-        && policy != return_value_policy::reference
-        && policy != return_value_policy::reference_internal) {
-        // SMART_HOLDER_WIP: IMPROVABLE: Error message.
-        throw cast_error("Invalid return_value_policy for unique_ptr.");
+    if (policy == return_value_policy::copy) {
+        throw cast_error("return_value_policy::copy is invalid for unique_ptr.");
     }
     if (!src) {
         return none().release();