Tri.declarative

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* `get_meta()` now collects extra arguments in a `Namespace` to get consistent override behaviour.

* `should_show` no longer accepts a callable as a valid return value. It will assert on this, because it's always a mistake.

* Added `evaluate_strict` and `evaluate_recursive_strict` that will not accept callables left over after the evaluation. If possible prefer these methods because they will stop the user of your library from making the mistake of not matching the given signature and ending up with an unevaluated callable in the output.

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* Fixed issues when Namespace contained a key called any of items, values, keys, or get

* Removed sorting on Namespace kwargs that isn't needed in python 3 anymore. The sorting also destroys the given order which can be surprising

* Removed old obsolete functions collect_namespaces, extract_subkeys, and setdefaults

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* Renamed module from `tri.declarative` to `tri_declarative`. This is a breaking change

* Dropped support for python2

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* Fixed `get_signature` cache to not pollute struct-like dicts

* New call_target semantics for class method shortcuts, this is a potential breaking change

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* Improved documentation output

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* Add get_members function to enable reuse of declarative attribute collection

* Add class_shortcut decorator to enable with_meta aware class shortcuts