Do no-type-slots dictoffset check as python lookup

Rather than just declaring it impossible.

Fixes cython#5696

Cython/Utility/ExtensionTypes.c

@@ -155,38 +155,36 @@ static int __Pyx_validate_bases_tuple(const char *type_name, Py_ssize_t dictoffs
 #endif
             return -1;
         }
-#if !CYTHON_USE_TYPE_SLOTS
-        if (dictoffset == 0) {
-            PyErr_Format(PyExc_TypeError,
-                "extension type '%s.200s': "
-                "unable to validate whether bases have a __dict__ "
-                "when CYTHON_USE_TYPE_SLOTS is off "
-                "(likely because you are building in the limited API). "
-                "Therefore, all extension types with multiple bases "
-                "must add 'cdef dict __dict__' in this compilation mode",
-                type_name);
-#if CYTHON_AVOID_BORROWED_REFS
-            Py_DECREF(b0);
-#endif
-            return -1;
-        }
-#else
-        if (dictoffset == 0 && b->tp_dictoffset)
+        if (dictoffset == 0)
         {
-            __Pyx_TypeName b_name = __Pyx_PyType_GetName(b);
-            PyErr_Format(PyExc_TypeError,
-                "extension type '%.200s' has no __dict__ slot, "
-                "but base type '" __Pyx_FMT_TYPENAME "' has: "
-                "either add 'cdef dict __dict__' to the extension type "
-                "or add '__slots__ = [...]' to the base type",
-                type_name, b_name);
-            __Pyx_DECREF_TypeName(b_name);
-#if CYTHON_AVOID_BORROWED_REFS
-            Py_DECREF(b0);
-#endif
-            return -1;
+            Py_ssize_t b_dictoffset = 0;
+#if CYTHON_USE_TYPE_SLOTS
+            b =  = b->tp_dictoffset;
+#else
+            PyObject *py_b_dictoffset = PyObject_GetAttrString((PyObject*)b, "__dictoffset__");
+            if (!py_b_dictoffset) goto dictoffset_return;
+            b_dictoffset = PyLong_AsSsize_t(py_b_dictoffset);
+            Py_DECREF(py_b_dictoffset);
+            if (b_dictoffset == -1 && PyErr_Occurred()) goto dictoffset_return;
+#endif
+            if (b_dictoffset) {
+                __Pyx_TypeName b_name = __Pyx_PyType_GetName(b);
+                PyErr_Format(PyExc_TypeError,
+                    "extension type '%.200s' has no __dict__ slot, "
+                    "but base type '" __Pyx_FMT_TYPENAME "' has: "
+                    "either add 'cdef dict __dict__' to the extension type "
+                    "or add '__slots__ = [...]' to the base type",
+                    type_name, b_name);
+                __Pyx_DECREF_TypeName(b_name);
+                {
+                  dictoffset_return:
+    #if CYTHON_AVOID_BORROWED_REFS
+                    Py_DECREF(b0);
+    #endif
+                    return -1;
+                }
+            }
         }
-#endif
 #if CYTHON_AVOID_BORROWED_REFS
         Py_DECREF(b0);
 #endif