Update dependency graphene to v2.1.9

Author: renovate[bot]

{{ cookiecutter.project_slug }}/server/requirements.txt

@@ -12,7 +12,7 @@ graphene-django==2.9.0
 
 # Technically graphene and graphql-core should not have to be specified here,
 # but running into a compatibility issue unless it is locked as pre 3.0
-graphene==2.1.8
+graphene==2.1.9
 graphql-core==2.3.1
 
 # Data stores