Make `graphql_schema` output deterministic

**Is your feature request related to a problem? Please describe.**

We have a CI job that checks that our schema file has been updated, by comparing the output of `graphql_schema` with the committed file. This intermittently fails due to the fact that the ordering of entries in the schema is non-deterministic.

**Describe the solution you'd like**

It would be great if the ordering of entries was deterministic - this would solve this issue for my team.

**Describe alternatives you've considered**

We can write a wrapper for the command, but it would be preferable if this were solved upstream.

**Additional context**

```
$ pip freeze | grep graphene
graphene==3.3
graphene-django==3.1.5
```