AZW3 Output: Fix handling of & < and > entities in the text. They were being incorrectly unescaped.

src/calibre/ebooks/mobi/writer8/skeleton.py

@@ -10,6 +10,7 @@
 import re
 from collections import namedtuple
 from functools import partial
+from xml.sax.saxutils import escape
 
 from lxml import etree
 
@@ -289,6 +290,7 @@ def step_into_tag(self, tag, chunks):
         self.chunk_selector = ('S', aid)
 
     def chunk_up_text(self, text):
+        text = escape(text)
         text = text.encode('utf-8')
         ans = []