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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (36af9c0).

Additional details and impacted files

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

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[juliosuas]

Accidentally closed when the source fork was deleted during a profile cleanup. Same commits restored here: #1342