userguide: dynamically determine copyright date

This uses the date of doc generation to determine the copyright date
for the trailing date. Based on Jeff Lucovsky solution.

(cherry picked from commit e0c8dba7ac)

doc/userguide/conf.py

@@ -16,6 +16,8 @@ import sys
 import os
 import shlex
 import re
+import subprocess
+import datetime
 
 on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
 
@@ -50,7 +52,8 @@ master_doc = 'index'
 
 # General information about the project.
 project = u'Suricata'
-copyright = u'2016-2019, OISF'
+end_year = datetime.datetime.now().date().year
+copyright = u'2016-{}, OISF'.format(end_year)
 author = u'OISF'
 
 # The version info for the project you're documenting, acts as replacement for