Release 2.0.0: Django 4.2-7.0 support with async-safe operations

[meteozond]

Major version upgrade with breaking changes and modern Django support:

Breaking Changes:

• Minimum Django version: 4.2 LTS (supports Django 4.2-5.2, experimental Django 6.0, tested with Django 7.0 dev)
• Minimum Python version: 3.10 (supports 3.10, 3.11, 3.12)
• Dropped Django 1.x, 2.x, 3.x support
• Dropped Python 2.7, 3.4-3.9 support
• Removed django-model-utils dependency

Async Support (New):

• Refactored from thread-local variables to contextvars for async-safe operation
• All deletion and query context handling now supports asyncio
• Context managers (deletion_context, show_all_context) are fully async-safe
• Works correctly with Django's async QuerySet API (acount, afirst, aget, etc.)
• Future-proof for Django 6.0 and 7.0 async expansion

New Features:

• Added Django System Check (W001) to detect problematic PermanentModel configurations
• Warns when PermanentModel has CASCADE ForeignKey to non-PermanentModel
• Comprehensive documentation (README.md)

Infrastructure:

• Migrated CI/CD from Travis CI to GitHub Actions
• All 33 tests passing with Django 4.2-7.0 dev - 100% pass rate, NO SKIPS
• Converted documentation from RST to Markdown
• Added explicit LICENSE file (BSD)

CI/CD Enhancements:

• Added automated PyPI publishing workflow (GitHub Actions)
• Automatically publishes to PyPI when version tag is pushed
• Creates GitHub releases with release notes
• Auto-updates CHANGES.md with next version template after release
• Improved test infrastructure with extras_require for test/dev dependencies
• Added TEST_VERBOSITY environment variable support for flexible test output
• Enhanced coverage reporting and Coveralls integration

Technical Improvements:

• Modernized codebase for Django 4.2+
• Removed Python 2/six dependencies
• Removed 15+ version compatibility checks
• Simplified QuerySet and Manager implementations
• Updated for Django 5.2+ API changes (Collector.delete, ForeignObject.get_extra_restriction, Signal)
• Enhanced async deletion and lazy reference handling

[meteozond]

• Cannot restore based on removed date MnogoByte/django-permanent#53
• get related object for all_objects  MnogoByte/django-permanent#75
• Error on django > 3 when django-utils-six is not installed MnogoByte/django-permanent#78
• Update deletion.py MnogoByte/django-permanent#79
• Installation instructions MnogoByte/django-permanent#69
• How is restore_on_create supposed to work? MnogoByte/django-permanent#66
• README: Improved description of how Permanent.restore_on_create works. MnogoByte/django-permanent#67
• Reorganize generic code MnogoByte/django-permanent#26
• Implement future imports to fit Django import code style MnogoByte/django-permanent#18