ci(release): Migrate to PyPI Trusted Publisher

[tony]

Summary

• Migrate PyPI publishing from API token to OIDC-based Trusted Publisher
• Enable package attestations for supply chain security
• Fix deprecated skip_existing parameter

Changes

• Add permissions block with id-token: write and attestations: write
• Remove user and password parameters from publish step
• Add attestations: true to enable cryptographic attestations
• Change skip_existing to skip-existing (kebab-case)

Setup Required

Before merging, configure the trusted publisher on PyPI:

1. Visit: https://pypi.org/manage/project/libtmux/settings/publishing/
2. Add publisher with:
   • Owner: tmux-python
   • Repository: libtmux
   • Workflow: tests.yml

Test plan

• Configure trusted publisher on PyPI
• Merge PR
• Create a release tag to verify publishing works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.68%. Comparing base (4df1128) to head (e7ab9c5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #615   +/-   ##
=======================================
  Coverage   45.68%   45.68%           
=======================================
  Files          22       22           
  Lines        2250     2250           
  Branches      360      360           
=======================================
  Hits         1028     1028           
  Misses       1079     1079           
  Partials      143      143           

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