sapling: migrate to python@3.13

[branchv]

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3.14 is unfortunately still broken

[cho-m]

Seems like something is broken with newer Python:

  ==> /opt/homebrew/Cellar/sapling/0.2.20260317-201835/bin/sl config --user ui.username Sapling <sapling@sapling-scm.com>
  SystemError: null argument to internal routine
  Fatal Python error: _PyImport_InitExternal: external importer setup failed
  Python runtime state: core initialized
  
  Current thread 0x00000001f052d0c0 (most recent call first):
    <no Python frame>

[branchv]

Ah, I only checked the build locally but not the test

Might need facebook/sapling#1251

[github-actions]

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