Drop pytz dependency in favor of Python's standard library timezone

[Copilot]

This PR removes the pytz dependency from django-dbbackup, aligning with Django's migration to Python's standard library zoneinfo as the default timezone implementation since Django 4.0.

Changes Made

• Removed pytz from project dependencies in pyproject.toml
• Updated test code to use datetime.timezone.utc instead of pytz.utc
• Updated changelog to document this breaking change

Background

Django 4.0 introduced zoneinfo as the default timezone implementation, moving away from pytz. The django-dbbackup package was still carrying pytz as a dependency despite only using it in a single test case for timezone-aware datetime testing.

Impact

This change:

• Reduces the package's dependency footprint
• Aligns with modern Django timezone handling practices
• Maintains full backward compatibility for end users
• Has no impact on the core backup/restore functionality

Testing

• ✅ All 242 unit tests pass across Python 3.12 and Django 4.2, 5.0, 5.1, 5.2
• ✅ Functional tests pass (database and media backup/restore workflows)
• ✅ Code linting and formatting checks pass
• ✅ Manual verification confirms dbbackup.utils.timestamp() works correctly with datetime.timezone.utc

The utils.timestamp() function already uses Django's timezone utilities (django.utils.timezone), so no changes were needed to the core functionality.

Fixes #611.

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[Copilot]

Pull Request Overview

This PR modernizes django-dbbackup's timezone handling by removing the pytz dependency and using Python's standard library datetime.timezone.utc instead. This aligns with Django 4.0+'s migration to zoneinfo as the default timezone implementation.

• Remove pytz dependency from project dependencies
• Update test code to use datetime.timezone.utc instead of pytz.utc
• Document the breaking change in the changelog

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
pyproject.toml	Removes pytz from the dependencies list
tests/test_utils.py	Updates import and test code to use standard library timezone
CHANGELOG.md	Documents the breaking change for the next major release