Port to Sphinx 8.0

[cjwatson]

What do these changes do?

The old intersphinx_mapping format has been removed; it must now map identifiers to (target, inventory) tuples.

Are there changes in behavior for the user?

No.

Checklist

• I think the code is well written
• [N/A] Unit tests for the changes exist
• Documentation reflects the changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.13%. Comparing base (3ad4302) to head (3a14699).
Report is 156 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #832   +/-   ##
=======================================
  Coverage   94.13%   94.13%           
=======================================
  Files          11       11           
  Lines         597      597           
  Branches       30       30           
=======================================
  Hits          562      562           
  Misses         26       26           
  Partials        9        9           

Flag	Coverage Δ
unit	94.13% <ø> (ø)

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