python3 compatibility fix
From [P3 what's new page](http://docs.python.org/3.0/whatsnew/3.0.html)
string.letters and its friends (string.lowercase and string.uppercase) are gone. Use string.ascii_letters etc. instead. (The reason for the removal is that string.letters and friends had locale-specific behavior, which is a bad idea for such attractively-named global “constants”.)
    
        | author | Catalin Costache <catacgc@gmail.com> | 
    
        | date | Mon, 23 Dec 2013 01:02:52 +0200 | 
    
    
        | parents | de2735512614 | 
    
        | children | f7ec69fb5eb5 | 
    
        | branches/tags | (none) | 
    
        | files | ffind | 
Changes
    
--- a/ffind	Sat Nov 24 13:41:51 2012 -0500
+++ b/ffind	Mon Dec 23 01:02:52 2013 +0200
@@ -699,7 +699,7 @@
 
     """
 
-    return not all(c.lower() in string.letters + '_-' for c in s)
+    return not all(c.lower() in string.ascii_letters + '_-' for c in s)
 
 
 def clean_ago_piece(n, unit):