Update FiniteDatetimeRange.days to require passing a TZ

It's possible to create mixed TZ FiniteDatetimeRange's.
It probably shouldn't be, but it currently is and this is
unlikely to change too soon.
See #192

This makes the FiniteDatetimeRange.days poorly defined.
For example, should the below range be 31 or 30 days?

RANGE = ranges.FiniteDatetimeRange(
    # This is also 2024-03-01T00:00:00 in TZ_UTC
    datetime.datetime(2024, 3, 1, tzinfo=TZ_LONDON),
    # This is 2024-04-01T00:00:00 in TZ_LONDON
    datetime.datetime(2024, 3, 31, hour=23, tzinfo=TZ_UTC),
)

This PR updates FiniteDatetimeRange.days to require passing
a TZ, to remove this ambiguity.

tests/test_ranges.py

@@ -13,6 +13,10 @@
 from xocto import localtime, ranges
 
 
+TZ_UTC = zoneinfo.ZoneInfo("UTC")
+TZ_LONDON = zoneinfo.ZoneInfo("Europe/London")
+
+
 @composite
 def valid_integer_range(draw):
     boundaries = draw(sampled_from(ranges.RangeBoundaries))
@@ -1278,6 +1282,23 @@ def test_errors_if_end_not_midnight(self):
 
             assert "End of range is not midnight-aligned" in str(exc_info.value)
 
+    class TestDays:
+        # This range crosses the DST boundary.
+        # At the start TZ_LONDON is equal to TZ_UTC.
+        # At the end TZ_LONDON is +1 to TZ_UTC.
+        RANGE = ranges.FiniteDatetimeRange(
+            # This is also 2024-03-01T00:00:00 in TZ_UTC
+            datetime.datetime(2024, 3, 1, tzinfo=TZ_LONDON),
+            # This is 2024-04-01T00:00:00 in TZ_LONDON
+            datetime.datetime(2024, 3, 31, hour=23, tzinfo=TZ_UTC),
+        )
+
+        def test_days_in_london(self):
+            assert self.RANGE.days(tz=TZ_LONDON) == 31
+
+        def test_days_in_utc(self):
+            assert self.RANGE.days(tz=TZ_UTC) == 30
+
 
 class TestAsFiniteDatetimePeriods:
     def test_converts(self):

xocto/ranges.py

@@ -901,12 +901,12 @@ def __and__(
     ) -> Optional["FiniteDatetimeRange"]:
         return self.intersection(other)
 
-    @property
-    def days(self) -> int:
+    def days(self, tz: datetime.tzinfo) -> int:
         """
         Return the number of days between the start and end of the range.
         """
-        return (self.end - self.start).days
+        range_ = self.localize(tz)
+        return (range_.end - range_.start).days
 
     @property
     def seconds(self) -> int: