Ignore AGPL link

This has been showing as broken more often than not in the recent week.

docs/conf.py

@@ -79,8 +79,9 @@
      # we have links to localhost for explanatory purposes; obviously
      # they will never work in the linkchecker
      r'^http://localhost:\d+',
-     # This URL will fail and return 403 (broken).
+     # These URLs are valid but return broken when using linkcheck. They can still be checked manually.
      r'^https://academic\.oup\.com/bioinformatics/article/34/23/4121/5001388',
+     r'https://www\.gnu\.org/licenses/agpl-3\.0\.en\.html',
 ]
 linkcheck_anchors_ignore_for_url = [
      # Github uses anchor-looking links for highlighting lines but