htmlproofer: ignore 403 codes

we are plagued by intermittent 403 results in CI.
note that we already have this change over in the docs repo at
scala/docs.scala-lang@b8117aa
so it should be fine here too.

Author: SethTisue

.github/workflows/ci.yaml

@@ -30,4 +30,4 @@ jobs:
       - name: Proofing (all links)
         if: github.event_name != 'pull_request'
         run: |
-          bundle exec htmlproofer ./_site/ --external_only --only-4xx --http-status-ignore "400,401,429" --empty-alt-ignore --allow-hash-href --url-ignore "/trends.google.com/,/pgp.mit.edu/,/www.oracle.com/,/scalafiddle.io/" --typhoeus-config='{"headers":{"Accept-Encoding":"gzip, deflate", "Accept":"application/xml,application/xhtml+xml,text/html;q=0.9, text/plain;q=0.8,image/png,*/*;q=0.5", "User-Agent":"Mozilla/5.0 (compatible; HTML Proofer/#{HTMLProofer::VERSION}; +https://github.com/gjtorikian/html-proofer)"}, "followlocation":"true", "connecttimeout":"10", "timeout":"30"}'
+          bundle exec htmlproofer ./_site/ --external_only --only-4xx --http-status-ignore "400,401,403,429" --empty-alt-ignore --allow-hash-href --url-ignore "/trends.google.com/,/pgp.mit.edu/,/www.oracle.com/,/scalafiddle.io/" --typhoeus-config='{"headers":{"Accept-Encoding":"gzip, deflate", "Accept":"application/xml,application/xhtml+xml,text/html;q=0.9, text/plain;q=0.8,image/png,*/*;q=0.5", "User-Agent":"Mozilla/5.0 (compatible; HTML Proofer/#{HTMLProofer::VERSION}; +https://github.com/gjtorikian/html-proofer)"}, "followlocation":"true", "connecttimeout":"10", "timeout":"30"}'