Drop support for django-filter < 2

It was only needed for Python 2.7

Author: ktosiek

docs/filtering.rst

@@ -2,9 +2,8 @@ Filtering
 =========
 
 Graphene integrates with
-`django-filter <https://django-filter.readthedocs.io/en/master/>`__ (2.x for
-Python 3 or 1.x for Python 2) to provide filtering of results. See the `usage
-documentation <https://django-filter.readthedocs.io/en/master/guide/usage.html#the-filter>`__
+`django-filter <https://django-filter.readthedocs.io/en/master/>`__ to provide filtering of results.
+See the `usage documentation <https://django-filter.readthedocs.io/en/master/guide/usage.html#the-filter>`__
 for details on the format for ``filter_fields``.
 
 This filtering is automatically available when implementing a ``relay.Node``.

graphene_django/filter/filterset.py

@@ -1,7 +1,7 @@
 import itertools
 
 from django.db import models
-from django_filters import Filter, MultipleChoiceFilter, VERSION
+from django_filters import Filter, MultipleChoiceFilter
 from django_filters.filterset import BaseFilterSet, FilterSet
 from django_filters.filterset import FILTER_FOR_DBFIELD_DEFAULTS
 
@@ -50,36 +50,6 @@ class GrapheneFilterSetMixin(BaseFilterSet):
     )
 
 
-# To support a Django 1.11 + Python 2.7 combination django-filter must be
-# < 2.x.x. To support the earlier version of django-filter, the
-# filter_for_reverse_field method must be present on GrapheneFilterSetMixin and
-# must not be present for later versions of django-filter.
-if VERSION[0] < 2:
-    from django.utils.text import capfirst
-
-    class GrapheneFilterSetMixinPython2(GrapheneFilterSetMixin):
-        @classmethod
-        def filter_for_reverse_field(cls, f, name):
-            """Handles retrieving filters for reverse relationships
-             We override the default implementation so that we can handle
-            Global IDs (the default implementation expects database
-            primary keys)
-            """
-            try:
-                rel = f.field.remote_field
-            except AttributeError:
-                rel = f.field.rel
-            default = {"name": name, "label": capfirst(rel.related_name)}
-            if rel.multiple:
-                # For to-many relationships
-                return GlobalIDMultipleChoiceFilter(**default)
-            else:
-                # For to-one relationships
-                return GlobalIDFilter(**default)
-
-    GrapheneFilterSetMixin = GrapheneFilterSetMixinPython2
-
-
 def setup_filterset(filterset_class):
     """ Wrap a provided filterset in Graphene-specific functionality
     """