Tri.token

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* Removed inheritance from FrozenStruct. Token instances are now plain objects.

* Fix hash value of ad-hoc created Token instances

* Fix pydantic type checking and string coercion of tokens

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* Raise clear exception message if a token is used in a dataclass and passed to asdict.

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* Fix bug where token instance was not a valid value for a pydantic dataclass or model (only string type was allowed).

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* TokenContainer is now usable within a pydantic dataclass or model.

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* Minor fix of missing reST escape.

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* Include tri.struct 4.x as possible requirement