handle DST properly #1209

[aurora324]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

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Description of Changes

Bug Fix

• Fixed the time zone calculation error in the shift() method during DST transitions.
• Original issue: Performing relativedelta operations directly on localized time caused errors during DST transition points.
• New solution: Convert times with time zones to UTC for calculations, then convert them back to the original time zone.

Code Changes

# Original code (problematic code)
current = self._datetime + relativedelta(**relative_kwargs)

# Modified code (fixed code)
original_tz = self.tzinfo
if self._datetime.tzinfo is not None:
    utc_dt = self._datetime.astimezone(dateutil_tz.UTC)
    current = utc_dt + relativedelta(**relative_kwargs)
    current = current.astimezone(original_tz)
else:
    current = self._datetime + relativedelta(**relative_kwargs)

[jadchaar]

Mind adding unit tests for this?