ci: add release-please bootstrap for master

[MilesCranmerBot]

Summary

• add a release-please workflow for master
• seed CHANGELOG.md from the existing GitHub releases, including the current v2.0.0a1 prerelease line
• add release-please-config.json and .release-please-manifest.json with bootstrap-sha set to the current master tip
• pin master to a forked release-please commit that preserves Python PEP 440 prerelease versions like 2.0.0a2

Notes

• keeps the existing .github/workflows/pypi_deploy.yml workflow unchanged
• continues to use the built-in GITHUB_TOKEN for Release Please; existing PyPI tokens remain in the PyPI deploy workflow
• uses versioning: prerelease so the current 2.0.0a* line continues to bump as prereleases

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.25%. Comparing base (95ffa50) to head (642ec6a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1162   +/-   ##
=======================================
  Coverage   94.25%   94.25%           
=======================================
  Files          21       21           
  Lines        1687     1687           
=======================================
  Hits         1590     1590           
  Misses         97       97           

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