Django-bread

Add Django 3.2 and Python 3.10 support

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* Switch from Travis to Github Actions. There are no functional
changes in the package, but I'm using this release to test the
Github Actions release workflow.

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* Drop specific version dependencies in setup.py for
django-filter and django-vanilla-views
* Update supported versions of Python and Django,
dropping Django 2.1 and adding 3.0 and 3.1,
and dropping Python 3.5 and 3.6 and adding 3.8 and 3.9.

Supported versions in this release are:

view permissions, before upgrading, make sure you've
migrated your users and groups that have "read_{model_name}"
permissions to also have "view_{model_name}". From 1.0.0 on, that's the
default permission a user needs to use the read views, because it's become the
standard Django permission for read-only access since Django 2.1.

If you're still on Django 2.0, don't upgrade django-bread until you
can get to at least Django 2.1. (Hopefully that's not the case, since
Django 2.0 has been out of support since April 1, 2019.)