Yay

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- Fix handling of paths on searchpath without a scheme.

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- Change how Django boxing works so that a .bind isn't required for the
boxing mechanism to work. This allows raw Django mdoels to be pushed
into Yay using the ``.add`` API from 0.0.41.

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- You can inject Python objects into the Yay Config object, as long as Yay
knows how to box them. If you had a Yay file like this::

hello: abc
result: ${hello}

And the following python code::

from yay.config import Config
c = Config()
c.load_uri(example_file)
c.add(dict(hello="xyx"))

Then the following assertion is true::

c.get() == dict(hello="xyz", result="xyz")

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- Fixes calling macros defined in other files.

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- Fix string builder so that configs containing secrets can be pickled.

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- This is a bugfix release to improve existing features, especially boxing of
foreign data.

- Move more code away from direct use of ``Boxed`` to ``BoxingFactory.box``

- Wrap any dictionary objects in a Mapping object so they can be merged with
other mappings that come from within Yay.