Fix Cinder's awaiter flag to not conflict with version tag

Summary:
`Py_TPFLAGS_HAVE_VERSION_TAG   (1UL << 18)` is defined on line 575 as `(1UL << 18)` so we can't use 18 here. Change to 21 which is unused.

This is causing a crash when subclassing a Cython class D60555759 and trying to copy the non-existent async methods here: https://www.internalfb.com/code/fbsource/[03a8cbcbe4d4f8d35cf0837de169925d564d479f]/third-party/python/3.12/Objects/typeobject.c?lines=3352

Reviewed By: yulong-zhang-1

Differential Revision: D60627205

fbshipit-source-id: d3d5998b39df88d9aa03c04539f7adfdf52f0121

Include/object.h

@@ -527,15 +527,15 @@ given type object has a specified feature.
 /* Objects behave like an unbound method */
 #define Py_TPFLAGS_METHOD_DESCRIPTOR (1UL << 17)
 
-/* Set if type's tp_as_async slot points to Ci_AsyncMethodsWithExtra */
-#define Ci_TPFLAGS_HAVE_AM_EXTRA (1UL << 18)
-
 /* Object has up-to-date type attribute cache */
 #define Py_TPFLAGS_VALID_VERSION_TAG  (1UL << 19)
 
 /* Type is abstract and cannot be instantiated */
 #define Py_TPFLAGS_IS_ABSTRACT (1UL << 20)
 
+/* Set if type's tp_as_async slot points to Ci_AsyncMethodsWithExtra */
+#define Ci_TPFLAGS_HAVE_AM_EXTRA (1UL << 21)
+
 // This undocumented flag gives certain built-ins their unique pattern-matching
 // behavior, which allows a single positional subpattern to match against the
 // subject itself (rather than a mapped attribute on it):