fix: add CHANGELOG.rst and enable semantic-release changelog

[farhan]

Set up automated changelog generation for python-semantic-release, part of the Open edX Python repo modernization effort: openedx/public-engineering#506

Changes

• Add CHANGELOG.rst with the .. changelog-insertion-marker that python-semantic-release updates on each release.
• Add [tool.semantic_release.changelog] config to pyproject.toml — mode = "update", insertion_flag = ".. changelog-insertion-marker", and default_templates pinning changelog_file = "CHANGELOG.rst" / output_format = "rst".
• Drop the changelog: "false" input from the Run Semantic Release step in release.yml so PSR generates the changelog on release.

Effect

On each release, PSR inserts the new version's notes at the marker in CHANGELOG.rst. The changelog: "false" flag and this feature are mutually exclusive, so the flag is removed.

Release note

Uses a fix: commit type so PSR cuts a release for this change.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.17%. Comparing base (b19a7a5) to head (08edf88).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #70   +/-   ##
=======================================
  Coverage   82.17%   82.17%           
=======================================
  Files          48       48           
  Lines        1419     1419           
  Branches      110      110           
=======================================
  Hits         1166     1166           
  Misses        221      221           
  Partials       32       32           

Flag	Coverage Δ
unittests	82.17% <ø> (ø)

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