don't check buffer.com links

The website appeared down a few days ago, but now even when it's up, it won't pass the link check. I can request the site locally with curl just fine, so I think they probably had a DDoS or something and now they employed an advanced protection. That often means they block IPs from ranges used by CI services.

Author: honzajavorek

conf.py

@@ -185,8 +185,9 @@
 
 linkcheck_ignore = [
     r'http://127.0.0.1:\d+',  # localhost
-    r'https://docs.google.com/.+',  # links to non-public documents
-    r'https://twitter.com/.+',  # Twitter prevents this kind of requests
+    r'https://docs.google.com/.+',  # links to private documents
+    r'https://twitter.com/.+',  # they prevent this kind of requests
+    r'https://(www\.)?buffer.com.*',  # they seem to prevent the CI's IP range
 ]
 sphinx_tabs_valid_builders = ['linkcheck']