linkcheck: replace 'linkcheck_allowed_redirects' default sentinel with boolean false value

[jayaddison]

Purpose

Patches the linkcheck module so that linkcheck_allowed_redirects is never assigned a value of None -- this was failing to typecheck against the allowed values for the config value when HTML themes were enabled (ref: #13462).

Specifically: this pull request updates the default value of linkcheck_allowed_redirects from the previous sentinel value to a new value of False.

Linkchecking and allowed redirections behaviour

When the linkcheck builder follows a hyperlink that redirects, the link is considering working when either of the following occur:

• The server issues a redirect back to the same URL (with any trailing-slashes removed).
• A matching {origin, destination} pattern pair for the server-issued redirect is found in linkcheck_allowed_redirects.

All other redirects are reported with the redirected status code.

When linkcheck_allowed_redirects is configured -- e.g. set to any other value than the default of False -- hyperlinks that encounter redirects with no matching matching {origin, destination} pattern pair in the configuration value also emit a warning-level log message.

References

• Resolves Possible regression in linkcheck_allowed_redirects default following PR 13452 #13462.
• Relates-to linkcheck only emits warnings on HTTP redirects if linkcheck_allowed_redirects is non-empty #13439.
• Tests should succeed both with and without Tests: uncover a quirk in our linkcheck tests #13466.

Edit: possible-values in this description requires redrafting; add a todo placeholder.
Edit: update proposed config value behaviours.
Edit: invert default value (was: true; now: false).
Edit: attempt to clarify this description.
Edit: describe the change-in-behaviour nearer the start of this description, reformat a few elements, and attempt to communicate the logic more clearly.
Edit: a correction about the existing default config value (sentinel), and a clarification about the described conditions under which redirected hyperlinks are considered working.

[pllim]

FWIW this PR turns our downstream job from red to green: astropy/sphinx-astropy#82

Thanks!

[jayaddison]

Thank you @pllim for testing it :)

[bsipocz]

Can we merge this please, so I don't keep getting the failing cron notifications 😄 ?

[jayaddison]

@AA-Turner ping for possible code review?