Remove the outdated documentation copyright year

https://github.com/python-ldap/python-ldap/pull/398

Doc/conf.py

@@ -10,6 +10,7 @@
 # All configuration values have a default value; values that are commented out
 # serve to show the default value.
 
+import datetime
 import sys
 import os
 
@@ -54,7 +55,7 @@
 
 # General substitutions.
 project = 'python-ldap'
-copyright = '2008-2017, python-ldap project team'
+copyright = 'python-ldap project team'
 
 # The default replacements for |version| and |release|, also used in various
 # other places throughout the built documents.