* change Sphinx version from 1.5.3 to 7.0.0
* change sphinx-autobuild version from 0.7.1 to 2021.3.14
* add pygments-graphql-lexer to docs/requirements.txt
* 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.