Rst2json

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- Drop support for Python 3.6
- Support Python 3.11
- Migrated from setuptools to hatch
- Declare the project unsupported going forwards

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- Support Python 3.10
- Update for Docutils 0.18.1, which is now the minimum required version

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- Added an `rst2json.core.get_docutils_config_files()` function for fetching
the list of config files Docutils will read from based on the environment
- Support Python 3.9
- Updated test cases for Docutils 0.17 and pinned Docutils version in test
environments to 0.17

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- **Breaking**: When not a string, the `format` argument to the `rst2json()`
function must now be an instance of a writer class, not an actual class
- Added `content.document_classes` and `content.subtitle_classes` fields to
output
- Added a `split_section_level` option for splitting the content body up by
sections

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- Fix failing tests

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- Added a convenience function, `rst2json()`, for converting reStructuredText
to JSON directly within Python
- Added information to the README on using the library's writer classes