Injectable

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* Fix Injectable failing to resolve complex/entangled imports

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* Fix optional injection bug when the namespace is empty

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* Fix the use of named args by the caller breaking autowired functions injection

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* Deprecate ``InjectionContainer::load`` in favor of ``load_injection_container``.
* Change default namespace name from ``"_GLOBAL"`` to ``"DEFAULT_NAMESPACE"``.
* Fix minor quirks with Python 3.7 and 3.8.
* Add tons of unit tests.
* Add ``reset_injection_container`` utility to ``injectable.testing``.

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* Include the ``injectable.testing`` utilities to ease mocking injectables.

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* ``InjectionContainer::load`` is more resilient against duplicated injectables
registering