Make str more subclassing friendly

Also, try to set the HAVE_NEWBUFFER flag more accurately

src/capi/abstract.cpp

@@ -135,7 +135,9 @@ extern "C" void PyBuffer_Release(Py_buffer* view) noexcept {
 
     PyObject* obj = view->obj;
     if (obj) {
-        assert(obj->cls == str_cls);
+        // This is a Pyston assert
+        assert(isSubclass(obj->cls, str_cls));
+
         if (obj && Py_TYPE(obj)->tp_as_buffer && Py_TYPE(obj)->tp_as_buffer->bf_releasebuffer)
             Py_TYPE(obj)->tp_as_buffer->bf_releasebuffer(obj, view);
         Py_XDECREF(obj);

src/runtime/objmodel.cpp

@@ -304,6 +304,9 @@ BoxedClass::BoxedClass(BoxedClass* base, gcvisit_func gc_visit, int attrs_offset
     tp_flags |= Py_TPFLAGS_HAVE_GC;
     tp_flags |= Py_TPFLAGS_HAVE_WEAKREFS;
 
+    if (base && (base->tp_flags & Py_TPFLAGS_HAVE_NEWBUFFER))
+        tp_flags |= Py_TPFLAGS_HAVE_NEWBUFFER;
+
     tp_base = base;
 
     if (tp_base) {

src/runtime/types.cpp

@@ -1320,6 +1320,8 @@ void setupRuntime() {
     tuple_cls->finishInitialization();
     list_cls->finishInitialization();
 
+    str_cls->tp_flags |= Py_TPFLAGS_HAVE_NEWBUFFER;
+
 
 
     module_cls = BoxedHeapClass::create(type_cls, object_cls, NULL, offsetof(BoxedModule, attrs), 0,

test/tests/str_subclassing.py

+class MyStr(str):
+    pass
+
+s = MyStr(1)
+print repr(s)
+
+import sys
+sys.stdout.write(s)