Use PyType_GetDict in Python 3.12+

Previously we were incorrectly accessing `tp_dict` in mixin classes when
defining a `Struct` class on Python 3.12+. This could result in a
segfault if a user tried to create a `Struct` class that also subclassed
from an interpreter builtin type (like `Exception`). Now we error nicely
instead.

msgspec/_core.c

@@ -57,6 +57,14 @@ ms_popcount(uint64_t i) { \
 #define SET_SIZE(obj, size) (((PyVarObject *)obj)->ob_size = size)
 #endif
 
+/* In Python 3.12+, tp_dict is NULL for some core types, PyType_GetDict returns
+ * a borrowed reference to the interpreter or cls mapping */
+#if PY312_PLUS
+#define MS_GET_TYPE_DICT(a) PyType_GetDict(a)
+#else
+#define MS_GET_TYPE_DICT(a) ((a)->tp_dict)
+#endif
+
 #if PY313_PLUS
 #define MS_UNICODE_EQ(a, b) (PyUnicode_Compare(a, b) == 0)
 #else
@@ -5527,10 +5535,9 @@ structmeta_collect_base(StructMetaInfo *info, MsgspecState *mod, PyObject *base)
 
  static const char *attrs[] = {"__init__", "__new__"};
  Py_ssize_t nattrs = 2;
+ PyObject *tp_dict = MS_GET_TYPE_DICT((PyTypeObject *)base);
  for (Py_ssize_t i = 0; i < nattrs; i++) {
- if (PyDict_GetItemString(
- ((PyTypeObject *)base)->tp_dict, attrs[i]) != NULL
- ) {
+ if (PyDict_GetItemString(tp_dict, attrs[i]) != NULL) {
  PyErr_Format(PyExc_TypeError, "Struct base classes cannot define %s", attrs[i]);
  return -1;
  }

tests/test_struct.py

@@ -246,6 +246,12 @@ def __new__(self):
  class Test(Struct, Mixin):
  pass
 
+ def test_mixin_builtin_type_errors(self):
+ with pytest.raises(TypeError):
+
+ class Test(Struct, Exception):
+ pass
+
 
 def test_struct_subclass_forbids_non_types():
  # Currently this failcase is handled by CPython's internals, but it's good