Python-box

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* Fixing 205 setdefault behavior with box_dots (thanks to Ivan Pepelnjak)

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* Adding py.typed for mypy support (thanks to Dominic)
* Adding testing for Python 3.10-dev
* Fixing 189 by adding mappings for mypy
* Fixing setdefault behavior with box_dots (thanks to ipcoder)
* Changing 193 how magic methods are handled with default_box (thanks to Rexbard)

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* Adding support for functions to box_recast (thanks to Jacob Hayes)
* Adding 181 support for extending or adding new items to list during `merge_update` (thanks to Marcos Dione)
* Fixing maintain stacktrace cause for BoxKeyError and BoxValueError (thanks to Jacob Hayes)
* Fixing 177 that emtpy yaml files raised errors instead of returning empty objects (thanks to Tim Schwenke)
* Fixing 171 that `popitems` wasn't first checking if box was frozen (thanks to Varun Madiath)
* Changing all files to LF line endings
* Removing duplicate `box_recast` calls (thanks to Jacob Hayes)
* Removing coveralls code coverage, due to repeated issues with service

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* Adding checks for frozen boxes to `pop`, `popitem` and `clear` (thanks to Varun Madiath)
* Fixing requirements-test.txt (thanks to Fabian Affolter)
* Fixing Flake8 conflicts with black (thanks to Varun Madiath)
* Fixing coveralls update (thanks to Varun Madiath)

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* Adding testing for Python 3.9
* Fixing 165 `box_dots` to work with `default_box`

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* Adding 152 `dotted` option for `items` function (thanks to ipcoder)
* Fixing 157 bug in box.set_default where value is dictionary, return the internal value and not detached temporary (thanks to Noam Graetz)
* Removing warnings on import if optional libraries are missing