Duration pickle and shallow copy lose year and month components #1023

Problem

Pickling or shallow-copying a Duration loses its calendar components even though its approximate total length remains equal.

import copy
import pickle
import pendulum

original = pendulum.duration(years=2, months=3, weeks=1, days=2)
restored = pickle.loads(pickle.dumps(original))
print(original.years, original.months)  # 2 3
print(restored.years, restored.months)  # 0 0
print(str(original))   # 2 years 3 months 1 week 2 days
print(str(restored))   # 118 weeks 3 days
print(copy.copy(original).years)  # 0

The inherited timedelta reduction serializes only its normalized day/second/microsecond fields, not the Duration instance state. Existing pickle coverage checks equality, so it doesn't catch the calendar information loss.

I plan to preserve the instance state alongside timedelta's existing reduction, with regressions for every supported pickle protocol, positive/negative/mixed-sign calendar components and shallow copying. Interval already defines its own reduction and should keep its existing behavior.

Reproduced from master with the pure-Python implementation on macOS / Python 3.12.13. Nineteen new regression cases fail before the fix. Checked existing duration pickle/copy issues and PRs for overlapping work.

Investigated with Codex assistance.