Xmlschema

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* Key/Unique/Keyref constraints partially rewritten
* Fixed ad issue with UCS-2/4 and maxunicode

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* UnicodeSubset class rewritten (more speed, less memory)
* Updated unicode_categories.json to Python 3.6 unicodedata
* Added XMLSchemaChildrenValidationError exception

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* Some bug fixes
* Code cleaning
* XSD components modules has been merged with schema's modules into 'validators' subpackage

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* Improved test scripts with a *SchemaObserver* class and test line arguments
* Full support for date and time XSD builtin types

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* Added identity constraints
* Some bug fix

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* Factories code moved to XsdComponent subclasses for simplify parsing and debugging
* All XSD components built from ElementTree elements with a lazy approach
* Implementation of the XSD validation modes ('strict'/'lax'/'skip') both for validating
schemas and for validating/decoding XML files
* Defined an XsdBaseComponent class as the common base class for all XSD components,
schemas and global maps
* Defined a ValidatorMixin for sharing a common API between validators/decoders classes
* Added built and validity checks for all XSD components