Make Timezone and FixedTimezone hashable

[Pebble32]

Timezone and FixedTimezone define __eq__ but not __hash__, so Python marks them unhashable and they cannot be used in sets or as dict keys:

>>> import pendulum
>>> {pendulum.timezone("Europe/Paris")}
TypeError: unhashable type: 'Timezone'

This adds __hash__ to both, hashing exactly what __eq__ compares (key for Timezone, _offset for FixedTimezone), so equal timezones hash equal. Hashing the string form would not be correct for FixedTimezone: its repr includes the name while equality ignores it.

Tests: test_hash and test_fixed_hash next to the existing equality tests, including two equal fixed timezones with different names collapsing to a single set entry.

Prompted by apache/airflow#73859, where Airflow currently works around this. Happy to add a changelog line if you want one.

Closed #1008, closed #1009 , closes #1019

[codspeed]

Merging this PR will not alter performance

✅ 1 untouched benchmark

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_{Comparing Pebble32:fix-timezone-hash (9c72264) with master (d3f44aa)}