Admin-extra-urls

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* add support Django 3.0
* add support Django 2.2
* dropped support Python <3.5
* dropped support Django <2.0

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* code cleanup

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* dropped support Python 2.x
* dropped support Django < 2.0
* add Django 2.1 support
* Updates _confirm_action utility to properly handle exceptions

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* Fixed the link css classes (issue 15)
* Fixed broken download url (issue 12)
* Fixed broken pypip.in badges (issue 11)

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* standard "post action redirect" now preserve existing filters
* check arguments of decorated methods
* fixes permission check using callable
* removed UploadMixin. It is now part of the demo app.