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 config setting are:

• linkcheck_allowed_redirects:{}: default follow redirects to attempt to resolve them, and emit warnings when redirects occur.
• linkcheck_allowed_redirects:_sentinel_lar: follow redirects to attempt to resolve them, and do not warn about them.
• linkcheck_allowed_redirects:{...}: follow redirects to attempt to resolve them, and warn about any that do not resolve to the expected destinations.

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.