Fix #807: handle pytz.FixedOffset in pendulum.instance()

[apoorvdarshan]

Summary

Fixes #807. pendulum.instance() raised AttributeError: 'NoneType' object has no attribute 'lower' for a datetime whose tzinfo is a pytz.FixedOffset:

import datetime, pytz, pendulum
pendulum.instance(datetime.datetime(2021, 2, 3, tzinfo=pytz.FixedOffset(60)))
# AttributeError: 'NoneType' object has no attribute 'lower'

Root cause

_safe_timezone() detects pytz timezones by the presence of a localize() method and then uses their .zone name:

elif hasattr(obj, "localize"):
    obj = obj.zone

Named pytz zones (e.g. America/New_York) expose a zone string, but fixed-offset zones (pytz.FixedOffset) have zone = None. That None was passed to timezone(None), which does name.lower() and crashes.

Fix

When a pytz zone has no zone name, fall back to its UTC offset (the same approach already used for other offset-only tzinfo objects further down the same branch). Named zones are unaffected.

Verification

• Added test_instance_with_pytz_fixed_offset (positive and negative offsets), guarded by pytest.importorskip("pytz") since pytz is not a test dependency. It reproduces the exact AttributeError on master and passes with this change.
• Full test suite: 1834 passed, 3 skipped (unchanged from baseline).
• Confirmed no regression for named pytz zones, pytz.UTC, zoneinfo, dateutil, and stdlib datetime.timezone.
• ruff check, ruff format --check, and mypy (strict) clean on the changed files.

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Disclosure: prepared with AI assistance; reviewed and verified locally against the test suite.

[Secrus]

If I understand it correctly, without pytz, this test will never execute. We are not installing pytz in our tests/pipelines. Hence, this test is pointless.

[Secrus]

Please drop the changelog entry - we don't add do "unreleased" in the changelog.

[codspeed-hq]

Merging this PR will not alter performance

✅ 1 untouched benchmark

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_{Comparing apoorvdarshan:fix-807-pytz-fixed-offset (47718d7) with master (cbab60c)}