feat: modernize repo to use uv and pyproject.toml

[farhan]

Important

PR implemented with the assistance of Claude Code, human-reviewed and improved before pushing to code review.

Summary

Modernize `openedx-webhooks` to uv + pyproject.toml (PEP 621/735) + src/ layout.

Part of openedx/public-engineering#506.

• Replace `setup.py`/`setup.cfg` with `pyproject.toml` (PEP 621 static metadata)
• Switch from pip-compile to `uv` with PEP 735 dependency groups; commit `uv.lock`
• Move the package into a `src/` layout
• Retain pylint/isort/pycodestyle as on master (ruff deferred to its own epic per #506); coverage config moved into `pyproject.toml`
• Update `tox.ini` to use `tox-uv` with `uv-venv-lock-runner` (newly introduced — master had no `tox.ini`)
• Update CI to use `astral-sh/setup-uv`; SHA-pin all actions; add `workflow_call` trigger

Removed

Deleted files: `setup.py`, `setup.cfg`, `requirements/`, `.coveragerc`

Removed Makefile targets:

Target	Reason
`check-setup.py`	Invoked `setup.py check`; `setup.py` no longer exists
`compile-requirements`	pip-compile replaced by `uv lock`; pip-tools no longer used
`install-dev-requirements`	pip-sync replaced by `uv sync`; pip-tools no longer used
`pip-compile`	pip-compile replaced by `uv lock`; pip-tools no longer used
`pip-compile-upgrade`	Superseded by `uv lock --upgrade` in the `upgrade` target

Not included/implemented

• `release.yml` / `python-semantic-release` — master had no PyPI publish workflow.

Versioning

[Static] `version = "0.1.0"` declared directly in `pyproject.toml` — master had no PyPI publish workflow, so `setuptools-scm` is not used and the version is bumped manually on each release tag.

Testing Notes

This PR has not been manually tested against the repo's own features. Testing relied on CI checks and local agent tooling (`make requirements`, `make lint`, `make test`, `python -m build`). Repo-owner is encouraged to run the repo's feature tests before merging.

Code reviewer notes:

• `tox.ini` is newly introduced (master had no tox config); all environments use `tox-uv` with `uv-venv-lock-runner` so tox environments are driven by `uv.lock` rather than re-resolving on each run.
• CI matrix now explicitly enumerates tox environments (`lint`, `mypy`, `docs`, `py312-test`) rather than running a single combined step, making individual failures easier to identify.
• The branch-protection required check name `Tests (ubuntu-latest, 3.12)` is preserved via a `job-name` matrix override on the `py312-test` entry — no admin access needed to update branch protection.
• `commitlint.yml` was already present on master before this PR — no behavioral change introduced by this migration.
• `codecov.yml` is newly added (header comment + reference URL only — no thresholds introduced).
• `upgrade` target now runs `uv lock --upgrade` instead of pip-compile; the new `requirements` target replaces `install-dev-requirements` with `uv sync --group dev`.

---

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.07%. Comparing base (6b9feb1) to head (c864a4d).

Files with missing lines	Patch %	Lines
src/openedx_webhooks/tasks/pr_tracking.py	50.00%	1 Missing and 1 partial ⚠️
src/openedx_webhooks/utils.py	75.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (6b9feb1) and HEAD (c864a4d). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (6b9feb1)	HEAD (c864a4d)
	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #437       +/-   ##
===========================================
- Coverage   89.84%   78.07%   -11.77%     
===========================================
  Files          38       19       -19     
  Lines        3103     1364     -1739     
  Branches      239      168       -71     
===========================================
- Hits         2788     1065     -1723     
+ Misses        281      273        -8     
+ Partials       34       26        -8     

Flag	Coverage Δ
unittests	78.07% <82.35%> (?)

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