Genson

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Modernization

* add support for Python versions up through 3.12
* remove support for old Python versions older than 3.7 since test dependencies no longer support them
* remove Python 2.7 support

* remove tests & test commands only relevant to Python 2.7
* remove backwards-compatibility from code

* enable running as a module (``python -m genson``)
* modernize package configuration (issue 68)
* Use a valid ``schema_uri`` in tests (issue 69)

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* add ``__version__`` attr to module and ``--version`` option to CLI tool
* add ``--encoding`` option to CLI tool that overrides default file encoding (fixes 47)

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* expose ``SchemaStrategy.__eq__()`` for extension
* add support for Python 3.8
* update Trove classifiers
* **Bugfix**: ``SchemaBuilder.__eq__()`` wasn't matching the ``$schema`` keyword correctly
* **Bugfix**: only activate empty ``required`` option when ``required`` is actualy empty

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* ``SchemaStrategies`` are now extendable, enabling custom ``SchemaBuilder`` classes.
* optimize ``__eq__`` logic

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* add support for Python 3.7
* drop support for Python 3.3
* drop support for JSON-Schema Draft 4 (because it doesn't allow empty ``required`` arrays)
* **Bugfix**: preserve empty ``required`` arrays (fixes 25)
* **Bugfix**: handle nested ``anyOf`` keywords (fixes 35)

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* add support for ``long`` integers in Python 2.7
* update test-skipping decorator to use standard version requirement strings