test: Ignore DESY Indico presentation URL during Sphinx linkcheck (#2230)

* Ignore URLs for https://indico.desy.de/event/22731 from Sphinx
  linkcheck tests as the URL
  https://indico.desy.de/event/22731/contributions/47953/ is frequently
  generating 403 Client Error.

Author: matthewfeickert

docs/conf.py

@@ -515,6 +515,8 @@ def setup(app):
     r'https://doi\.org/10\.31526/.*',
     # https://doi.org/10.1051/epjconf/x DOI URLs will periodically generate 500 Server Error
     r'https://doi\.org/10\.1051/epjconf/.*',
+    # https://indico.desy.de/event/22731/contributions/47953/ is frequently generating 403 Client Error
+    r'https://indico.desy.de/event/22731/.*',
     # tags for a release won't exist until it is made, but the release notes
     # and ReadTheDocs need to reference them
     r'https://github.com/scikit-hep/pyhf/releases/tag/.*',