Fix BMeshElemSeq.sort(key=None) argument

Allow passing key=None to BMesh element sorting callback,
matching the doc-string & Python's list sort.

source/blender/python/bmesh/bmesh_py_types.cc

@@ -3006,6 +3006,9 @@ static PyObject *bpy_bmelemseq_sort(BPy_BMElemSeq *self, PyObject *args, PyObjec
     {
       return nullptr;
     }
+    if (keyfunc == Py_None) {
+      keyfunc = nullptr;
+    }
   }
 
   if (keyfunc != nullptr && !PyCallable_Check(keyfunc)) {