Work around constraint in python 3.4 when using HMAC

https://bugs.python.org/issue18240

Signed-off-by: Alexander Clouter <alex@digriz.org.uk>

offlineimap/imapserver.py

@@ -198,7 +198,7 @@ class IMAPServer(object):
         challenge = response.strip()
         self.ui.debug('imap', '__md5handler: got challenge %s'% challenge)
 
-        passwd = self.__getpassword()
+        passwd = self.__getpassword().encode('ascii')
         retval = self.username + ' ' + hmac.new(passwd, challenge).hexdigest()
         self.ui.debug('imap', '__md5handler: returning %s'% retval)
         return retval