Make the writer datetime test independent of local timezone

[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 is a test-only change; it does not alter production serialization or the separate orjson migration in #29. No Windows/OpenBSD run was performed locally.

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