conf.py: add help.github.com to linkcheck_ignore

Problem: sphinx linkcheck is getting a 403 Forbidden on the github
reference in spec_1.rst, though the link is tested working from a
web browser (Chrome).

Add this URL to linkcheck_ignore since it is causing CI to fail.

Author: grondo

conf.py

@@ -63,6 +63,10 @@
 master_doc = 'index'
 source_suffix = '.rst'
 
+linkcheck_ignore = [
+    "https://help.github.com/en/pull-requests", # 403 Forbidden
+]
+
 
 # -- Options for HTML output -------------------------------------------------