Django-http-fallback-storage

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- Dropped compatibility with Python 2.

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- Fixed the package to automatically install the ``requests``
dependency.
- Reformatted the code using `black <https://github.com/ambv/black>`__.

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- Reverted back to `print()`-ing by default. Set
``FALLBACK_STORAGE_LOGGING`` to ``True`` if ``http_fallback_storage``
should use the ``logging`` module instead. The reason for this is that
the ``logging`` behavior is a bit less obvious in that the reason for
a hanging development server is only visible if ``LOGGING`` is
configured correctly (which isn't hard to do, but still is something
that is overlooked too often).
- Added timeouts to downloads.
- Added the ``FALLBACK_STORAGE_SKIP`` setting which allows specifying a
regular expression which, if it matches the filename, skips the fallback.
If set to ``FALLBACK_STORAGE_SKIP = r'^test$'``, this avoids problems
introduced e.g. by `easy-thumbnails
<https://github.com/SmileyChris/easy-thumbnails/blob/c4483dc44cb748749df420d9cd1f57fb4fac469b/easy_thumbnails/utils.pyL67>`_.

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- Removed the cache again. It lead to strange errors with accessing
closed files.
- Switched from print() statements to logging. If you want the old
behavior you should use ``http_fallback_storage.LOGGING`` as your
Django ``LOGGING`` configuration (or merge the logging configuration
dicts).

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- Cache answers a bit. This is especially useful with non-existing
files.

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- Only write response data to the disk if HTTP status code is 200.
- Only access response data if there actually was a response.
- Avoid ``u''`` prefixes on strings printed to the console.