Elementpath

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* Extend root concept to subtrees used as root (e.g. XSD 1.1 assertions)
* Begin XPath 3.1 implementation adding XPathMap and XPathArray

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* Fix of descendant path operator (issue 51)
* Add support for Python 3.11

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* Transition to full XPath node implementation (more memory usage but
better control and overall faster)
* Add etree.py module with a safe XML parser (ported from xmlschema)

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* Fix unary path step operator (issue 46)
* Fix sphinx warnings *'reference target not found'* (issue 45)

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* Include PR 43 with fixes for `XPathContext.iter_siblings()` (issues 42 and 44)

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* Fix for failed floats equality tests (issue 41)
* Static typing tested with mypy==0.950