Handle edge case with C extension metaclasses.

If both a metaclass and an instance of it are defined in C extensions,
the metaclass doesn't have attrs_offset != 0 or tp_dictoffset != 0
which causes the call to PyType_Ready on the instance to fail.

src/capi/typeobject.cpp

@@ -3317,8 +3317,24 @@ extern "C" int PyType_Ready(PyTypeObject* cls) noexcept {
     cls->gc_visit = &conservativeGCHandler;
     cls->is_user_defined = true;
 
-    // this should get automatically initialized to 0 on this path:
-    assert(cls->attrs_offset == 0);
+
+    if (!cls->instancesHaveHCAttrs() && cls->tp_base) {
+        // These doesn't get copied in inherit_slots like other slots do.
+        if (cls->tp_base->instancesHaveHCAttrs()) {
+            cls->attrs_offset = cls->tp_base->attrs_offset;
+        }
+
+        // Example of when this code path could be reached and needs to be:
+        // If this class is a metaclass defined in a C extension, chances are that some of its
+        // instances may be hardcoded in the C extension as well. Those instances will call
+        // PyType_Ready and expect their class (this metaclass) to have a place to put attributes.
+        // e.g. CTypes does this.
+        bool is_metaclass = PyType_IsSubtype(cls, type_cls);
+        assert(!is_metaclass || cls->instancesHaveHCAttrs() || cls->instancesHaveDictAttrs());
+    } else {
+        // this should get automatically initialized to 0 on this path:
+        assert(cls->attrs_offset == 0);
+    }
 
     if (Py_TPFLAGS_BASE_EXC_SUBCLASS & cls->tp_flags) {
         exception_types.push_back(cls);

src/runtime/types.h

@@ -197,7 +197,7 @@ public:
     // Analogous to tp_dictoffset
     // A class should have at most of one attrs_offset and tp_dictoffset be nonzero.
     // (But having nonzero attrs_offset here would map to having nonzero tp_dictoffset in CPython)
-    const int attrs_offset;
+    int attrs_offset;
 
     bool instancesHaveHCAttrs() { return attrs_offset != 0; }
     bool instancesHaveDictAttrs() { return tp_dictoffset != 0; }