offlineimap/ files singleton-comparison

This patch change these errors in the offlineimap folder

C0121: Comparison to None should be 'expr is None' (singleton-comparison)
C0121: Comparison to None should be 'expr is not None' (singleton-comparison)

offlineimap/CustomConfig.py

@@ -140,7 +140,7 @@ class CustomConfigParser(SafeConfigParser):
 
         Returns transformed string."""
 
-        if string == None:
+        if string is None:
             return None
         for f in transforms:
             string = f(string)