fixed custom columns handling on dispose

cps/db.py

@@ -1,4 +1,3 @@
-#!/usr/bin/env python
 # -*- coding: utf-8 -*-
 
 #  This file is part of the Calibre-Web (https://github.com/janeczku/calibre-web)
@@ -418,7 +417,7 @@ def dispose():
 
     for attr in list(Books.__dict__.keys()):
         if attr.startswith("custom_column_"):
-            delattr(Books, attr)
+            setattr(Books, attr, None)
 
     for db_class in cc_classes.values():
         Base.metadata.remove(db_class.__table__)