* merge master into v3
* fix order_by snake casing by checking if value is None, switch executor to execution_context_class since schema.execute no longer supports executor
* fix linting by removing duplicate defintion and test of convert_form_field_to_string_list
* Remove Python 2 support
* Upgrade Python & Django versions
* Remove unsupported Django versions
* Remove unsupported Python versions
* Add Python 3.8
* Drop support for django-filter < 2
* Update LoginRequiredMixin doc link
* Remove redundant import
* Resolve RemovedInDjango40Warning warnings
* gql/graphene-django/graphene_django/tests/test_converter.py:175:
RemovedInDjango40Warning: django.utils.translation.ugettext_lazy() is
deprecated in favor of django.utils.translation.gettext_lazy().
* graphene-django/graphene_django/utils/utils.py:28:
RemovedInDjango40Warning: force_text() is deprecated in favor of
force_str().
* No need to use unicode strings with Python3
* Remove singledispatch dependency
singledispatch is inluded with Python >= 3.4, no need for external
package.
* Include setup.py in black formatting
* Add new flake8 plugins and update errors to look for
* Fix duplicate test name
* Don't use mutable data structure
* Install all dev dependencies for flake8 and black tox envs
* Update travis and tox
* Use xenial distribution
* Don't install coveralls twice
* Add black and flake8 tox commands
* Remove Python 3.5 test for Django master
* Fix indent
* Ignore migrations
* Remove black for now
* Run black formatting (#668)
* Run black format
* Update makefile
* Add black to travis build
