Fix Timestamp.from_datetime() precision for far-future datetimes

[gaoflow]

Timestamp.from_datetime() computes the whole-second part from the float datetime.timestamp(). A float64 cannot hold microsecond precision for datetimes far from the epoch, so timestamp() rounds the seconds up while the exact microsecond is still used for the nanoseconds. The result is a Timestamp one second in the future, and near datetime.max it raises OverflowError:

>>> import datetime as dt
>>> from msgpack.ext import Timestamp
>>> d = dt.datetime(3000, 1, 1, 0, 0, 0, 999999, tzinfo=dt.timezone.utc)
>>> Timestamp.from_datetime(d).to_datetime()
datetime.datetime(3000, 1, 1, 0, 0, 1, 999999, tzinfo=datetime.timezone.utc)   # +1 second
>>> Timestamp.from_datetime(dt.datetime(9999, 12, 31, 23, 59, 59, 999999, tzinfo=dt.timezone.utc))
OverflowError: date value out of range

The Cython packer already uses integer timedelta arithmetic and is correct. This makes the pure-Python from_datetime() do the same, so the two paths agree and the round-trip invariant from_datetime(d).to_datetime() == d holds for far-future datetimes. Naive datetimes keep their existing local-time interpretation (matching datetime.timestamp()).

Follow-up to #662, which fixed the pre-epoch rounding direction in the same method.

Existing tests are unchanged; I added a regression test covering the far-future round-trip, the exact seconds/nanoseconds, agreement with packing the datetime directly, and the former OverflowError case. Full suite is green on both the C-extension and pure-Python paths.

[Copilot]

Pull request overview

This PR fixes msgpack.ext.Timestamp.from_datetime() so it computes the whole-second component using integer timedelta arithmetic instead of datetime.timestamp() (float), preventing off-by-one-second rounding and OverflowError for far-future datetimes while keeping naive-datetime “local time” semantics consistent with datetime.timestamp().

Changes:

• Switch Timestamp.from_datetime() to derive seconds/nanoseconds from dt - epoch using integer math (and handle naive datetimes via astimezone() to preserve existing behavior).
• Introduce a shared module-level UTC epoch constant and reuse it in to_datetime().
• Add regression tests covering far-future round-trip precision, exact seconds/nanoseconds, packer agreement, and the prior OverflowError case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
msgpack/ext.py	Reworks Timestamp.from_datetime() to avoid float timestamp precision/overflow and reuses a module-level UTC epoch in datetime conversions.
test/test_timestamp.py	Adds regression tests to ensure correct far-future datetime round-trips and consistency with datetime packing.

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