widen mapping-like object support when merging them into a kwargs dict

Cython/Utility/FunctionArguments.c

@@ -317,7 +317,22 @@ static int __Pyx_MergeKeywords(PyObject *kwdict, PyObject *source_mapping) {
     Py_ssize_t orig_length, ppos = 0;
 
     iter = __Pyx_dict_iterator(source_mapping, 0, PYIDENT("items"), &orig_length, &source_is_dict);
-    if (unlikely(!iter)) goto bad;
+    if (unlikely(!iter)) {
+        // slow fallback: try converting to dict, then iterate
+        PyObject *args;
+        if (!PyErr_ExceptionMatches(PyExc_AttributeError)) goto bad;
+        PyErr_Clear();
+        args = PyTuple_Pack(1, source_mapping);
+        if (likely(args)) {
+            PyObject *fallback = PyObject_Call((PyObject*)&PyDict_Type, args, NULL);
+            Py_DECREF(args);
+            if (likely(fallback)) {
+                iter = __Pyx_dict_iterator(fallback, 1, PYIDENT("items"), &orig_length, &source_is_dict);
+                Py_DECREF(fallback);
+            }
+        }
+        if (unlikely(!iter)) goto bad;
+    }
 
     while (1) {
         result = __Pyx_dict_iter_next(iter, orig_length, &ppos, &key, &value, NULL, source_is_dict);