set the number of retries when running linkcheck to 5

Retrying > once should increase the specificity of the check a lot.

doc/source/conf.py

@@ -272,6 +272,10 @@ def setup(app):
 
 # IOP times out on check but the link verified and correct.
 linkcheck_ignore = [r'https://iopscience.iop.org/1367-2630/16/6/063065/article']
+# By default linkcheck only tries once, but experience has shown us that this test is
+# very non-specific for detecting dead links. Trying a few times should significantly
+# lower the probability of false positives.
+linkcheck_retries = 5
 
 nitpick_ignore = [('py:class', 'Warning'), ('py:class', 'Exception'),
                   ('py:class', 'object'), ('py:class', 'tuple'),