Infra: Fix footnote warnings and allow Sphinx 8.1+

[hugovk]

So we can revert #4044.

I don't think there's a way to ignore specific warnings in Sphinx?

The current warnings:

• pep-0352.rst:279: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0376.rst:610: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0376.rst:613: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0386.rst:501: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0421.rst:469: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0421.rst:475: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0421.rst:490: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0421.rst:493: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0472.rst:626: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0472.rst:629: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0472.rst:632: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0504.rst:359: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0504.rst:362: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0512.rst:969: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0512.rst:972: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0512.rst:975: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0512.rst:978: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0512.rst:981: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0512.rst:984: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0512.rst:987: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0512.rst:996: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0512.rst:998: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0512.rst:1000: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0516.rst:455: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0516.rst:458: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0518.rst:519: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0518.rst:540: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0523.rst:371: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0523.rst:374: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0583.rst:783: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0583.rst:790: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0583.rst:794: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0583.rst:798: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0583.rst:810: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0583.rst:814: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0615.rst:938: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0703.rst:1829: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0703.rst:1875: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0725.rst:667: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-0747.rst:611: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0747.rst:614: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-0747.rst:621: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-3108.rst:1131: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-3108.rst:1140: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-3108.rst:1152: WARNING: Footnote [#] is not referenced. [ref.footnote]
pep-3108.rst:1162: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-3126.rst:349: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-3141.rst:522: WARNING: Footnote [#] is not referenced. [ref.footnote]
• pep-3146.rst:1299: WARNING: Footnote [#] is not referenced. [ref.footnote]