Update to Sphinx 9

[A5rocks]

I just released a new version of sphinxcontrib-trio so hopefully this works now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (0124394) to head (21250b1).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3384   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19414        19415    +1     
  Branches         1318         1318           
===============================================
+ Hits            19414        19415    +1     

Files with missing lines	Coverage Δ
src/trio/_core/_run.py	100.00000% <100.00000%> (ø)
src/trio/_tools/gen_exports.py	100.00000% <ø> (ø)

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