Tri.token

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* Added requirement to set `__override__=True` parameter for token that shadow token from base class.

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* Changed metaclass handling to make PyCharm understand it

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* First released version on github

* Added documentation

* Cleanup of build machinery

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* Added documentation generation sort customisation

* Added python 3 support

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* Move dependency tri.cache.memoize from requirements.txt to test_requirements.txt. It is only used
for regression testing.