Django-hashedfilenamestorage

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* Bump Django dependency requirement again

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* Bump Django dependency requirement to avoid vulnerable Django versions

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* Support Django 2.1+
* Remove duplicate hash calculation
* Always lower filename extensions

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* get_available_filename() now returns the default filename instead of raising
an exception, for compatibility with other packages (overridden by the hashed
filename on save)

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* Fix bug for bytes content
* Update setup.py and tox to support Django 2.0 for python version >= 3.4

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* Specify django <2.0 in setup.py