Fix linkcheck

Apparently, GitHub now adds the README HTML on a project page
dynamically through JS, which prevents Sphinx's link checker from
finding anchors into the README.

Author: jeanas

source/conf.py

@@ -109,6 +109,12 @@
     "https://www.breezy-vcs.org/*",
 ]
 linkcheck_retries = 5
+# Ignore anchors for links to GitHub project pages -- GitHub adds anchors from
+# README.md headings through JavaScript, so Sphinx's linkcheck can't find them
+# in the HTML.
+linkcheck_anchors_ignore_for_url = [
+    r"https://github\.com/",
+]
 
 # -- Options for extlinks ----------------------------------------------------------
 # https://www.sphinx-doc.org/en/master/usage/extensions/extlinks.html#configuration