Xmlschema

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* Add support for Python 3.12
* Detach content iteration methods from ModelVisitor

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* Fix mixed content extension with empty content (issue 337)
* Fix lru_cache() usage on global maps caching

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* Fix mixed content extension without explicit mixed attribute (issue 334)

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* Refine string serialization of XML resources and data elements
* Switch to use elementpath v4
* Fix sequence_type property for XSD types
* Remove *XsdElement.get_attribute()*: unused and doesn't work as expected

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* Fix *schema_path* usage in `XMLSchemaBase.iter_errors()`
* Add *allow_empty* option to `XMLSchemaBase` validation API

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* Add *to_etree()* to document API
* Improve generic encoding with wildcards
* Clean document API and schema decoding