Add joint calendar support: Calendar accepts a list of calendar types

[DMZ22]

Implements #228 — joint (multi-market) calendars for business-day adjustment, e.g. a swap with a USD SOFR floating leg and a EUR fixed leg that must avoid both US and TARGET holidays.

Design

Calendar now also accepts a list (or tuple) of CalendarTypes:

cal = Calendar([CalendarTypes.UNITED_STATES, CalendarTypes.TARGET])
cal.adjust(dt, BusDayAdjustTypes.MODIFIED_FOLLOWING)

• A date is a holiday if it is a holiday in any constituent calendar, so a business day must be a business day in every market — the standard join-holidays convention (same as QuantLib's JointCalendar).
• No new enum member. Existing tests (and likely user code) iterate list(CalendarTypes) and construct Calendar(t) for each member, so adding a JOINT member would break them. Instead, for a joint calendar cal_type holds the constituent types as a tuple, which never compares equal to any CalendarTypes member — so every existing code path (the NONE/WEEKEND fast paths, adjust, fast_adjust, add_business_days, get_holiday_list) works unchanged because they all funnel through is_business_day / is_holiday.
• A one-element or duplicates-only list collapses to the equivalent plain calendar; an empty list or a non-CalendarTypes entry raises FinError, matching the existing validation style.
• str()/repr() render as JOINT(UNITED_STATES,TARGET).

Scope

The change is deliberately confined to Calendar plus tests. Schedule and the product classes still annotate cal_type: CalendarTypes; if this API looks right to you, I'm happy to follow up with a small PR relaxing those annotations/checks so a list can be passed straight through Schedule and the swap constructors.

Testing

• 9 new test functions (13 test cases) in unit_tests/test_FinCalendar.py, including: a day-by-day sweep over 2020–2023 asserting the joint calendar's business days are exactly the intersection of the single calendars'; adjustment chaining across consecutive UK-only holidays (Good Friday → Easter Monday 2021, where a US-only calendar stays put but US+UK must jump to the Tuesday); fast_adjust ≡ adjust for joint calendars across all conventions; add_business_days round-trips; get_holiday_list union; collapse/dedup semantics; str/repr; and error cases.
• unit_tests/test_FinCalendar.py: 285 passed.
• Full unit suite on Python 3.11: 675 passed, with 17 collection errors that are identical on current master (multi-line f-strings, a Python-3.12-only feature, in modules unrelated to this change — your CI runs 3.13 so it doesn't see this; I'll file it as a separate issue since pyproject.toml declares >=3.10).
• pylint on the changed file: 10.00/10.