ci: build sdist with python -m build for a PEP 625-compliant filename

[wbarnha]

Description

The v0.12.0 publish job failed uploading the sdist to PyPI
(run):

400 Filename 'faust-streaming-0.12.0.tar.gz' is invalid,
    should be 'faust_streaming-0.12.0.tar.gz'.

All wheels uploaded fine (faust_streaming-0.12.0-*.whl, 200 OK) — only the
sdist was rejected.

Root cause

The build_sdist job built the tarball with the deprecated
python3 setup.py sdist under an old setuptools>=30.3.0 floor, which emits
the legacy hyphenated distribution filename faust-streaming-0.12.0.tar.gz.
PyPI now enforces PEP 625 and requires the
normalized name (underscores): faust_streaming-0.12.0.tar.gz.

Fix

• Build the sdist with python -m build --sdist instead of
  setup.py sdist.
• Raise the [build-system] setuptools floor to >=69, the version that
  normalizes sdist filenames per PEP 625.

The wheels were unaffected (cibuildwheel already emits the normalized name), so
no wheel-side change is needed. The publish step already sets
skip-existing: true, so a re-run will skip the already-uploaded 0.12.0 wheels
and upload only the (now correctly-named) sdist.

Verification

Local python -m build --sdist now produces:

faust_streaming-<version>.tar.gz   ✅  (was faust-streaming-<version>.tar.gz)

Note on recovering the v0.12.0 sdist

v0.12.0's wheels are already on PyPI; only its sdist is missing. Because a
workflow re-run uses the workflow file at the tagged commit (still the old
one), the cleanest way to get a correct sdist published is to cut v0.12.1
with this fix (wheels + sdist rebuild fresh; skip-existing handles any
overlap). Alternatively, build the sdist locally with python -m build --sdist
and twine upload it to the existing 0.12.0 release.

🤖 Generated with Claude Code

---

Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (3f8e190) to head (edf1592).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #712   +/-   ##
=======================================
  Coverage   94.15%   94.15%           
=======================================
  Files         104      104           
  Lines       11136    11136           
  Branches     1201     1201           
=======================================
  Hits        10485    10485           
  Misses        550      550           
  Partials      101      101           

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