Safe check Schema for Graphene v3

[KingDarBoja]

While testing locally the new server-core, I noticed I had to do this on my schema:

Notice the graphql_schema part, I don't remember having to use it but since we assert on run_http_query function that it MUST be a GraphQLSchema, now we are forced to use the attribute graphql_schema of the Schema type.

@jkimbo suggested at the Slack channel to add a little compatibility layer to ensure it will work for both cases:

if not a graphql-core schema class then check if the object has an attribute called graphql_schema and use that instead.

This issue was made as remainder.

[KingDarBoja]

Closing as fixed at #54 by adding a note at each server integration regarding this topic.