v3.1.3

Patch release GraphQL-core v3.1.3, based on GraphQL.js v15.2.0.

This release includes the following improvements:

• Python 3.9 is now officially suported by GraphQL-core.
• build_schema: allow to reference introspection types
• Custom validation rules for "no schema introspection" and "no deprecated"
• Added execute_sync() as synchronous version of execute()
• Support deep copy of GraphQL schema (#100)
• build_ast_schema now sets internal enum values (#111)
• Do not double-wrap resolver errors any more (#106)
• Use newer Sphinx version, fix for various autodoc issues (#104)
• Fix example in docstring of GraphQLUnionType (#105)
• Let default resolver check for Mapping instead of dict (#102)

Thanks to everyone who helped with their feedback, particularly

@berekuk, @charmasaur, @Checho3388, @jstlaurent and @pmantica1.