Add copy function for GrapheneGraphQLType

Fixes https://github.com/graphql-python/graphene/issues/1333

First found when using  with a Django project. Django's  tries to create a copy of a GrapheneGraphQLType and raises this KeyError

graphene/types/definitions.py

@@ -20,6 +20,11 @@ class GrapheneGraphQLType:
         self.graphene_type = kwargs.pop("graphene_type")
         super(GrapheneGraphQLType, self).__init__(*args, **kwargs)
 
+    def __copy__(self):
+        result = GrapheneGraphQLType(graphene_type=self.graphene_type)
+        result.__dict__.update(self.__dict__)
+        return result
+
 
 class GrapheneInterfaceType(GrapheneGraphQLType, GraphQLInterfaceType):
     pass

graphene/types/tests/test_definition.py

@@ -1,4 +1,7 @@
+import copy
+
 from ..argument import Argument
+from ..definitions import GrapheneGraphQLType
 from ..enum import Enum
 from ..field import Field
 from ..inputfield import InputField
@@ -312,3 +315,15 @@ def test_does_not_mutate_passed_field_definitions():
         pass
 
     assert TestInputObject1._meta.fields == TestInputObject2._meta.fields
+
+def test_graphene_graphql_type_can_be_copied():
+    class Query(ObjectType):
+        field = String()
+
+        def resolve_field(self, info):
+            return ''
+
+    schema = Schema(query=Query)
+    query_type_copy = copy.copy(schema.graphql_schema.query_type)
+    assert query_type_copy.__dict__ == schema.graphql_schema.query_type.__dict__
+    assert isinstance(schema.graphql_schema.query_type, GrapheneGraphQLType)