bpo-43541: Fix PyEval_EvalCodeEx() regression (GH-24918)

* Remove an assertion which required CO_NEWLOCALS and CO_OPTIMIZED
  code flags. It is ok to call this function on a code with these
  flags set.
* Fix reference counting on builtins: remove Py_DECREF().
  Fix regression introduced in the
  commit 46496f9d12582bf11f4911ad0f23315d6f277907.

Add also a comment to document that _PyEval_BuiltinsFromGlobals()
returns a borrowed reference.
diff --git a/Objects/frameobject.c b/Objects/frameobject.c
index 056d42a..a1413d7 100644
--- a/Objects/frameobject.c
+++ b/Objects/frameobject.c
@@ -847,7 +847,7 @@ PyFrameObject*
 PyFrame_New(PyThreadState *tstate, PyCodeObject *code,
             PyObject *globals, PyObject *locals)
 {
-    PyObject *builtins = _PyEval_BuiltinsFromGlobals(tstate, globals);
+    PyObject *builtins = _PyEval_BuiltinsFromGlobals(tstate, globals); // borrowed ref
     if (builtins == NULL) {
         return NULL;
     }