upb: use the new reflection headers

Reflection headers now live in upb/reflection/ so update our include statements to
reflect this. (See what I did there?)

PiperOrigin-RevId: 474596615

python/message.c

diff --git a/python/message.c b/python/message.c
index e67bf23..1752922 100644
--- a/python/message.c
+++ b/python/message.c
@@ -32,8 +32,8 @@
 #include "python/extension_dict.h"
 #include "python/map.h"
 #include "python/repeated.h"
-#include "upb/def.h"
-#include "upb/reflection.h"
+#include "upb/reflection/def.h"
+#include "upb/reflection/message.h"
 #include "upb/text_encode.h"
 #include "upb/util/required_fields.h"