[saitakarcesme]

Fixes #6. test_writer_bad_message calls timestamp() on a naive datetime, so its expected bytes depend on the host timezone. The two hard-coded timestamps only cover some environments; adding another timestamp as in the closed #7 would leave the underlying problem.

Use a UTC-aware JsonDatetime and keep a single timestamp expectation (1546304461), while retaining the existing empty-output allowance for the standard JSON fallback. Instantiate the subclass directly instead of permanently replacing the global datetime.datetime class.

Validation on Python 3.13.5:

• Before the change, the test passes in UTC and America/New_York but fails in Europe/Luxembourg, Asia/Kathmandu and Pacific/Chatham.
• After the change, all 27 tests pass in each of those five timezones with both ujson 6.0.0 and the standard-library JSON fallback (10 separate processes). Also verified datetime.datetime retains its original identity after each suite run.
• Repository-wide pylint (10.00/10), pycodestyle, pyflakes and git diff --check pass.

This changes tests and macOS CI only; it does not alter production serialization or the separate orjson migration in #29. No Windows/OpenBSD run was performed locally.

CI follow-up:

• The old macOS jobs requested the retired macos-13 runner and were cancelled without executing any steps.
• Switch only the runner label to macos-15-intel, preserving the x64 architecture and Python 3.8/3.9/3.10 matrix.
• YAML semantic comparison confirms only the runner label changed. git diff --check passes, and all 27 tests pass on Linux / Python 3.13.15 with TZ=Europe/Luxembourg. GitHub macOS execution is still required.

AI disclosure: prepared and tested by OpenAI Codex acting on behalf of @saitakarcesme. No independent human review is claimed.

[saitakarcesme]

Fixed the macOS CI blocker in a separate commit on this branch. The previous Mac tests run shows all three jobs cancelled with no runner assigned and no steps executed; each requested macos-13.

GitHub has retired that image and documents macos-15-intel as the standard Intel replacement. Changed only that runner label, preserving the x64 architecture, Python 3.8/3.9/3.10 matrix, test commands and timeout.

Validation: YAML parses and its only semantic difference is the runner label; git diff --check passes; all 27 tests pass locally on Linux / Python 3.13.15 with TZ=Europe/Luxembourg. The previous Linux, Windows and static-analysis workflows passed. The new macOS run must still execute on GitHub; no local macOS result is claimed.

The new Mac, Linux, Windows and static-analysis runs are now awaiting fork-workflow approval. Could a maintainer approve them, starting with the new Mac run?

Fix and follow-up prepared with Codex assistance.