Add ISO 8601 Duration support

[veeceey]

Adds a Duration class that handles ISO 8601 duration strings (the P1Y2M3DT4H5M6S format). Came up in #757.

Supports parsing, formatting, and doing math with Arrow objects:

from arrow import Duration

# Parse
d = Duration.parse("P1Y2M3DT4H5M6S")

# Format
str(d)  # "P1Y2M3DT4H5M6S"

# Use with Arrow
import arrow
arw = arrow.get("2020-01-01")
arw + Duration(years=1, months=2)  # 2021-03-01

# Convert
d.to_timedelta()
d.to_relativedelta()
Duration.from_timedelta(timedelta(days=5, hours=3))

Handles weeks (P2W), fractional values (PT1.5S), and the full date+time combination. Also supports duration arithmetic (add/subtract/negate) and equality/hashing.

39 new tests covering parsing, formatting, roundtrips, arithmetic, conversions, edge cases, and error handling.

Closes #757

[veeceey]

Test Results

All 39 new tests pass, and all 1940 existing tests continue to pass:

$ python3 -m pytest tests/ -x -o "addopts=" --timeout=60
======================= 1940 passed, 1 skipped in 2.96s ========================

$ python3 -m pytest tests/test_duration.py -xvs -o "addopts="
tests/test_duration.py::TestDurationParse::test_full_duration PASSED
tests/test_duration.py::TestDurationParse::test_date_only PASSED
tests/test_duration.py::TestDurationParse::test_time_only PASSED
tests/test_duration.py::TestDurationParse::test_weeks PASSED
tests/test_duration.py::TestDurationParse::test_days_only PASSED
tests/test_duration.py::TestDurationParse::test_hours_only PASSED
tests/test_duration.py::TestDurationParse::test_minutes_only PASSED
tests/test_duration.py::TestDurationParse::test_seconds_only PASSED
tests/test_duration.py::TestDurationParse::test_fractional_seconds PASSED
tests/test_duration.py::TestDurationParse::test_fractional_hours PASSED
tests/test_duration.py::TestDurationParse::test_years_months PASSED
tests/test_duration.py::TestDurationParse::test_invalid_empty PASSED
tests/test_duration.py::TestDurationParse::test_invalid_no_p PASSED
tests/test_duration.py::TestDurationParse::test_invalid_only_p PASSED
tests/test_duration.py::TestDurationParse::test_invalid_only_pt PASSED
tests/test_duration.py::TestDurationParse::test_invalid_garbage PASSED
tests/test_duration.py::TestDurationFormat::test_full_format PASSED
tests/test_duration.py::TestDurationFormat::test_date_only_format PASSED
tests/test_duration.py::TestDurationFormat::test_time_only_format PASSED
tests/test_duration.py::TestDurationFormat::test_weeks_format PASSED
tests/test_duration.py::TestDurationFormat::test_zero_duration PASSED
tests/test_duration.py::TestDurationFormat::test_roundtrip PASSED
tests/test_duration.py::TestDurationFormat::test_isoformat_method PASSED
tests/test_duration.py::TestDurationArithmetic::test_add_durations PASSED
tests/test_duration.py::TestDurationArithmetic::test_sub_durations PASSED
tests/test_duration.py::TestDurationArithmetic::test_negate PASSED
tests/test_duration.py::TestDurationArithmetic::test_add_to_arrow PASSED
tests/test_duration.py::TestDurationArithmetic::test_add_days_to_arrow PASSED
tests/test_duration.py::TestDurationArithmetic::test_add_time_to_arrow PASSED
tests/test_duration.py::TestDurationEquality::test_equal PASSED
tests/test_duration.py::TestDurationEquality::test_not_equal PASSED
tests/test_duration.py::TestDurationEquality::test_hash PASSED
tests/test_duration.py::TestDurationConversions::test_to_timedelta PASSED
tests/test_duration.py::TestDurationConversions::test_to_relativedelta PASSED
tests/test_duration.py::TestDurationConversions::test_from_timedelta PASSED
tests/test_duration.py::TestDurationRepr::test_repr_full PASSED
tests/test_duration.py::TestDurationRepr::test_repr_empty PASSED
tests/test_duration.py::TestDurationRepr::test_repr_partial PASSED
tests/test_duration.py::TestDurationImport::test_importable_from_arrow PASSED
============================== 39 passed in 0.18s ==============================

Manual test:

>>> from arrow import Duration
>>> d = Duration.parse("P1Y2M3DT4H5M6S")
>>> d
Duration(years=1, months=2, days=3, hours=4, minutes=5, seconds=6)
>>> str(d)
'P1Y2M3DT4H5M6S'
>>> import arrow
>>> arrow.get("2020-01-01") + Duration(years=1, months=2)
<Arrow [2021-03-01T00:00:00+00:00]>

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b423717) to head (44379cf).

Additional details and impacted files

@@            Coverage Diff             @@
##            master     #1251    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           10        11     +1     
  Lines         2315      2420   +105     
  Branches       358       379    +21     
==========================================
+ Hits          2315      2420   +105     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[veeceey]

any chance this could get reviewed? happy to make changes

[veeceey]

Pushed additional tests to cover the missing lines flagged by codecov - NotImplemented returns, microsecond handling, years/months in to_timedelta, and fractional formatting. Should bring patch coverage up to 100%.