Address PR review feedback: use unused tp_flags bit and reword test comment

- Change CPy_TPFLAGS_MYPYC_COMPILED from bit 20 (Py_TPFLAGS_IS_ABSTRACT) to
  bit 21, which is unused across all CPython versions (3.8-3.14)
- Reword test comment to not claim direct struct access, since a runtime test
  cannot prove which code path was taken

Author: VaggelisD

mypyc/lib-rt/mypyc_util.h

@@ -149,7 +149,7 @@ typedef PyObject CPyModule;
 // Flag bit set on all mypyc-compiled types. Used to distinguish compiled
 // subclasses (safe for direct struct access) from interpreted subclasses
 // (need PyObject_GenericGetAttr fallback) in allow_interpreted_subclasses mode.
-#define CPy_TPFLAGS_MYPYC_COMPILED (1UL << 20)
+#define CPy_TPFLAGS_MYPYC_COMPILED (1UL << 21)
 
 typedef void (*CPyVTableItem)(void);
 

mypyc/test-data/run-classes.test

@@ -5849,8 +5849,7 @@ assert s.check_flag(), "compiled method doesn't see subclass override"
 
 [case testCompiledSubclassAttrAccessWithAllowInterpretedSubclasses]
 # Test that compiled subclasses of a class with allow_interpreted_subclasses=True
-# can correctly access parent instance attributes via direct struct access
-# (not falling back to PyObject_GenericGetAttr).
+# can still correctly access parent instance attributes.
 from mypy_extensions import mypyc_attr
 
 @mypyc_attr(allow_interpreted_subclasses=True)