build(tox): migrate from pip to uv via tox-uv

[sentry-junior]

Description

• add uv and tox-uv to manage python envs and packages instead of pip
• use astral-sh/setup-uv action in CI instead of setup-python, uv always uses python3.13
• except for 3.6 and 3.7, all python versions now go through tox-uv
• 3.6 and 3.7 have their own containers which uv picks up through UV_PYTHON_REFERENCE
• the SDK is now installed as part of deps and not via package so uv resolves all deps in a single pass, this was necessary since uv resolution is stricter than pip
• some other pins were necessary to make CI pass

TODO

• pre-releases are currently handled with UV_PRERELEASE=if-necessary-or-explicit and explicit pinning of transitive deps, maybe it makes sense to split off pre-releases in a separate action where we can have UV_PRERELEASE=all
• coverage (which is a mess anyway)

[github-actions]

Codecov Results 📊

✅ 13 passed | Total: 13 | Pass Rate: 100% | Execution Time: 5.85s

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 16266 uncovered lines.

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Generated by Codecov Action

[sentry-warden]

RuntimeError raised in background thread is silently swallowed, allowing beat process to leak

When the pidfile never appears within pidfile_timeout, the RuntimeError is raised inside the kill_beat background thread — Python does not propagate thread exceptions to the caller, so run_beat continues and beat_instance.run() is never terminated, defeating the leak-prevention goal. Consider signalling the failure via a threading.Event or checking t.is_alive() / t.join() with a timeout after beat_instance.run() returns, or using concurrent.futures.Future to surface the exception.

Evidence

• kill_beat is launched via threading.Thread(target=kill_beat, ...) in run_beat (line 56).
• t.join() is never called; run_beat proceeds directly to beat_instance.run().
• In CPython, an unhandled exception in a Thread target is printed to stderr but does not propagate to the spawning thread.
• If the timeout fires and RuntimeError is raised at line 34, the killer thread exits silently, beat_instance.run() blocks indefinitely, and the beat process is never sent SIGTERM.
• The PR's stated goal ("so slow process startup doesn't race the killer into a FileNotFoundError that would leak a running beat") is therefore not achieved in the timeout branch.

_{Identified by Warden code-review · XNE-GLL}