fix: preserve instance locale in datetime.astimezone()

[patchwright]

Problem

jdatetime.datetime.astimezone(tz) silently drops the instance locale and resets it to the global default.

>>> import datetime, jdatetime
>>> jdatetime.set_locale('en_US')
>>> teh = datetime.timezone(datetime.timedelta(hours=3, minutes=30))
>>> dt = jdatetime.datetime(1397, 8, 17, 7, 54, 28, tzinfo=teh, locale='fa_IR')
>>> dt.astimezone(datetime.timezone.utc).locale
'en_US'

Expected 'fa_IR'. Confirmed on main HEAD 4b461d1.

Root cause

astimezone rebuilds the result with datetime.fromgregorian(datetime=gdt) and does not pass the instance locale (jdatetime/__init__.py:1176). Every sibling that round-trips through fromgregorian passes it: __add__ (1009), __sub__ (1022), replace (1002), combine (860). astimezone is the only outlier.

Fix

Pass locale=self.locale, matching the siblings. One line, no API change.

-        return datetime.fromgregorian(datetime=gdt)
+        return datetime.fromgregorian(datetime=gdt, locale=self.locale)

How to test

Added test_astimezone_keeps_locale, next to test_replace_keeps_date_locale. It fails on main with AssertionError: 'en_US' != 'fa_IR' and passes with this change. Real output:

before: astimezone().locale = 'en_US'
after:  astimezone().locale = 'fa_IR'

The full suite is green apart from a pre-existing test_with_fa_locale that fails with locale.Error: unsupported locale setting on machines missing the fa_IR.UTF-8 system locale. It fails identically on unpatched main, so it is unrelated to this change. A CHANGELOG entry is included.

Backward compatibility

No breaking changes. After astimezone, the instance locale is preserved, consistent with replace, combine, __add__, and __sub__.

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Assisted-by: Claude (code generation, reviewed and tested locally)