Remove linkcheck_ignore when Sphinx linkchecker stops failing on GitHub URLs

[kmpaul]

As referenced in #260:

This is now fixed with the merge of #259. However, it is a hack, so I'm going to create a standing issue to remove the linkcheck_ignore Sphinx option when sphinx-doc/sphinx#10343 is resolved, if ever.

Originally posted by @kmpaul in #260 (comment)

The linkcheck_ignore option is a complete hack. Something about the GitHub documentation URLs leads to Sphinx's link checker to fail with a 403 Client Error. This shouldn't happen. I have created an issue in the Sphinx repository here:

sphinx-doc/sphinx#10343

When that issue is resolved, then we should remove the linkcheck_ignore line from the _config.yml file.

[dopplershift]

As noted in the linked sphinx issue, it seems to be that the docs site is doing some kind of user agent filtering--it remains to be seen what they do about it (github/docs#17042) In the meantime, you can give the Sphinx linkchecker custom headers to pass on a per-host basis. I've got this working for MetPy in conf.py. #262 is my attempt to convert that config to yaml.