Move pickle implementation into the serialize module

And make it work on both py2 and py3. Note that unpickling of data
between python major versions is not likely to work well.

src/calibre/library/database.py

@@ -5,11 +5,12 @@ __copyright__ = '2008, Kovid Goyal <kovid at kovidgoyal.net>'
 Backend that implements storage of ebooks in an sqlite database.
 '''
 import sqlite3 as sqlite
-import datetime, re, cPickle, sre_constants
+import datetime, re, sre_constants
 from zlib import compress, decompress
 
 from calibre.ebooks.metadata import MetaInformation
 from calibre.ebooks.metadata import string_to_authors
+from calibre.utils.serialize import pickle_loads, pickle_dumps
 from calibre import isbytestring
 from polyglot.builtins import unicode_type
 
@@ -1090,7 +1091,7 @@ ALTER TABLE books ADD COLUMN isbn TEXT DEFAULT "" COLLATE NOCASE;
     def conversion_options(self, id, format):
         data = self.conn.get('SELECT data FROM conversion_options WHERE book=? AND format=?', (id, format.upper()), all=False)
         if data:
-            return cPickle.loads(str(data))
+            return pickle_loads(bytes(data))
         return None
 
     def has_conversion_options(self, ids, format='PIPE'):
@@ -1166,7 +1167,7 @@ ALTER TABLE books ADD COLUMN isbn TEXT DEFAULT "" COLLATE NOCASE;
             self.set_tags(id, val.split(','), append=False)
 
     def set_conversion_options(self, id, format, options):
-        data = sqlite.Binary(cPickle.dumps(options, -1))
+        data = sqlite.Binary(pickle_dumps(options))
         oid = self.conn.get('SELECT id FROM conversion_options WHERE book=? AND format=?', (id, format.upper()), all=False)
         if oid:
             self.conn.execute('UPDATE conversion_options SET data=? WHERE id=?', (data, oid))

src/calibre/utils/serialize.py

@@ -5,6 +5,7 @@
 from __future__ import absolute_import, division, print_function, unicode_literals
 
 from polyglot.builtins import unicode_type
+from calibre.constants import ispy3
 
 
 MSGPACK_MIME = 'application/x-msgpack'
@@ -114,3 +115,24 @@ def msgpack_loads(dump):
 def json_loads(data):
     import json
     return json.loads(data, object_hook=json_decoder)
+
+
+if ispy3:
+
+    def pickle_dumps(data):
+        import pickle
+        return pickle.dumps(data, -1)
+
+    def pickle_loads(dump):
+        import pickle
+        return pickle.loads(dump, encoding='utf-8')
+
+else:
+
+    def pickle_dumps(data):
+        import cPickle as pickle
+        return pickle.dumps(data, -1)
+
+    def pickle_loads(dump):
+        import cPickle as pickle
+        return pickle.loads(dump)