Undo attr.h, detail/class.h changes made under PR pybind#3923. See python/cpython#92678 for background.

Author: rwgk

include/pybind11/attr.h

@@ -345,11 +345,7 @@ struct type_record {
 
         bases.append((PyObject *) base_info->type);
 
-#if PY_VERSION_HEX < 0x030B0000
         dynamic_attr |= base_info->type->tp_dictoffset != 0;
-#else
-        dynamic_attr |= (base_info->type->tp_flags & Py_TPFLAGS_MANAGED_DICT) != 0;
-#endif
 
         if (caster) {
             base_info->implicit_casts.emplace_back(type, caster);

include/pybind11/detail/class.h

@@ -549,12 +549,8 @@ extern "C" inline int pybind11_clear(PyObject *self) {
 inline void enable_dynamic_attributes(PyHeapTypeObject *heap_type) {
     auto *type = &heap_type->ht_type;
     type->tp_flags |= Py_TPFLAGS_HAVE_GC;
-#if PY_VERSION_HEX < 0x030B0000
     type->tp_dictoffset = type->tp_basicsize;           // place dict at the end
     type->tp_basicsize += (ssize_t) sizeof(PyObject *); // and allocate enough space for it
-#else
-    type->tp_flags |= Py_TPFLAGS_MANAGED_DICT;
-#endif
     type->tp_traverse = pybind11_traverse;
     type->tp_clear = pybind11_clear;