fix(types): use Union[Arrow, dt_datetime] for span_range and interval start/end params

[juliosuas]

Problem

Arrow.span_range() and Arrow.interval() accept both Arrow and datetime objects for their start/end parameters (they pass through fromdatetime() internally), but their type hints only declare dt_datetime.

This means type checkers (mypy, pyright) raise false errors when passing Arrow objects directly:

start = arrow.now()
end = arrow.now().shift(hours=24)
list(arrow.Arrow.span_range('hour', start, end))  # ← mypy: error: Argument 2 to 'span_range' has incompatible type 'Arrow'; expected 'datetime'

Fix

Arrow.range() already uses the correct type Union['Arrow', dt_datetime] for its start/end params. This PR applies the same fix to span_range() and interval() for consistency.

# Before
start: dt_datetime,
end: dt_datetime,

# After  
start: Union['Arrow', dt_datetime],
end: Union['Arrow', dt_datetime],

Fixes #1210.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b423717) to head (1e96211).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1267   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2315           
  Branches       358       358           
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  Hits          2315      2315           

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[juliosuas]

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