Elementpath

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* Add XPath 3.0 support
* Better use of lxml.etree features
* Full coverage of W3C tests
* Drop support for Python 3.6

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* Fix type annotations and going strict on parsers and other public classes
* Add XPathConstructor token class (subclass of XPathFunction)
* Last release for Python 3.6

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* Make ElementProtocol and LxmlElementProtocol runtime checkable (only for Python 3.8+)
* Type annotations for all package public APIs

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* Add LxmlElementProtocol
* Add pytest env to tox.ini (test issue 39)

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* Add inline type annotations check support
* Add structural Protocol based type checks (effective for Python 3.8+)

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* Add Python 3.10 in Tox and CI tests
* Apply __slots__ to TDOP and regex classes