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

[juliosuas]

Fixes #1210.

Arrow.span_range() and Arrow.interval() accept start and end bounds like
Arrow.range() does, but their annotations only allowed datetime.

This updates those annotations to accept Arrow | datetime and normalizes
Arrow bounds before passing them through fromdatetime().

The test dependency markers also skip two optional test-only dependencies on
Windows PyPy. The affected dateparser regression test already uses
pytest.importorskip("dateparser"), and pre-commit is only needed by the
lint environment.

Tests:

tox -e lint
tox -e py314

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Restored after an accidental fork deletion. Same commits as #1267.