Pack datetime subclasses as timestamps unless strict_types is set - #755

With datetime=True and the default strict_types=False, the C Packer rejects a subclass of datetime.datetime, while the Python fallback packs it as a Timestamp. When a default callback is supplied, the C Packer calls it instead. This makes serialization depend on which backend is loaded.

The datetime checks now follow strict_types, as the other type checks do. Aware subclasses pack as timestamps, and naive subclasses raise the same ValueError as the fallback. Valid timedelta subclasses returned by subtraction are accepted. strict_types=True and datetime=False retain the existing subclass handling. Applications that intentionally serialize datetime subclasses through default can use strict_types=True to preserve that behavior.

For example, both backends now pack this value as d7ff64361d20661e3a1d:

import datetime
import msgpack

class DT(datetime.datetime):
    pass

value = DT(2024, 4, 16, 8, 43, 9, 420317, tzinfo=datetime.timezone.utc)
msgpack.packb(value, datetime=True)

The regressions cover timestamp bytes and round trips, callback dispatch, naive values, strict and disabled-datetime controls, and subtraction returning a timedelta subclass. A C-only test also checks that an invalid subtraction result raises TypeError; fallback behavior for invalid overrides is unchanged.

Validated on CPython 3.12.3 / Linux x86_64 with Cython 3.3.0:

• Tests alone on the pinned base: three expected C-extension failures; fallback passes with the C-only test skipped.
• Relaxing only the datetime checks still rejects a valid timedelta subclass; the complete patch passes all new cases.
• Full C-extension suite: 151 passed, 1 skipped.
• Full fallback suite: 147 passed, 5 skipped.
• Ruff 0.17.2 check and format produced empty diffs; source and wheel builds passed.

Other Python versions, operating systems, PyPy and free-threaded builds were not run locally.

Refs #683, finding 16.