Fix for python3

This fixes an attribute error

flask_simpleldap/__init__.py

@@ -2,6 +2,7 @@
 import re
 from functools import wraps
 import ldap
+import sys
 from ldap import filter as ldap_filter
 from flask import abort, current_app, g, make_response, redirect, url_for, \
     request
@@ -149,6 +150,9 @@ class LDAP(object):
             return
         try:
             conn = self.initialize
+            if sys.version_info[0] > 2:
+                conn.simple_bind_s(user_dn, password)
+            else:
                 conn.simple_bind_s(user_dn.decode('utf-8'), password)
             return True
         except ldap.LDAPError: