linkcheck: disallow 'None' value for 'linkcheck_allowed_redirects' setting

[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).

The range of possible values for the linkcheck_allowed_redirects config setting are:

• False (new default): report all redirects using the redirected (non-success) report code.
• {} (new): warn about all redirects and report them as redirections instead of success.
• { ... }: existing logic; permit a selection of regex-based source-to-destination redirections to be considered successful.

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).

[pllim]

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

Thanks!

[jayaddison]

Thank you @pllim for testing it :)