feat: add restriction for python 3

Add a restriction for python 3 so that installing this software with python 2 isn't possible.

Should fix GH-10.

gitea_auto_update/update.py

@@ -7,6 +7,7 @@ All rights reserved.
 License: GNU General Public License
 '''
 import os
+import sys
 import logging
 import configparser
 import fire
@@ -74,8 +75,12 @@ def updater(settings='settings.ini'):
                   config.get('Gitea', 'tmpDir'),
                   config.get('Gitea', 'system'))
 
+
 def main():
+    if not sys.version_info[0] == 3:
+        sys.exit("Sorry, Python 2 is not supported. Please update to Python 3.")
     fire.Fire(updater)
 
+
 if __name__ == '__main__':
   main()