ignore URL for link check

to make this pass:
https://github.com/fangohr/introduction-to-python-for-computational-science-and-engineering/runs/4691847458?check_suite_focus=true#step:10:109

Author: fangohr

book/_config.yml

@@ -59,3 +59,4 @@ sphinx:
   config:
     linkcheck_anchors: false
     linkcheck_workers: 10
+    linkcheck_ignore        : https://jacow.org/icalepcs2019/papers/tucpr02.pdf