fix(toolkit): a copied or unpickled DecimalTimestamp loses orig_len on Python 3.15 #1647

On Python 3.15.0, a copy.deepcopy and/or pickle round trip of pcapkit.toolkit.dpkt.DecimalTimestamp loses orig_len. The clone compares equal to the original, but its (exact, resolution, orig_len) is (Decimal('1500000000.000774789'), 1000000000, None); the original's is (..., 78). Timestamp, the float form, keeps orig_len. Python 3.11-3.14 pass.

Repro: tests/toolkit/test_engine_pcap_trace_runtime.py::DPKTTimestampTests::test_copies_and_arithmetic_keep_working, subtest DecimalTimestamp. It failed on #1645's Integration Python 3.15 leg (job 115150227943, run 38353246360). That leg is the first blocking 3.15 run; the old schedule-only 3.15 compat job was allowed to fail, so nothing caught this before.

Expected: a copied or unpickled DecimalTimestamp keeps exact, resolution and orig_len on every supported Python. The likely cause is a change to decimal.Decimal's reduce/copy protocol in 3.15; it will be established before the fix.