v3.1.5

Patch release GraphQL-core v3.1.5, based on GraphQL.js v15.4.0.

This release includes the following changes and imrovements:

• Default parse_literal of GraphQLScalarType now handles variables
• build_ast_schema now matches order of default types and directives
• Return formatted errors in formatted execution result (fixes #129)
• The is_deprecated property is now deprecated itself
• Input fields and arguments can now be marked as deprecated
• Handle case when MapAsyncIterator is cancelled (#131)
• Improve return type for complete_list_value (#132)
• Replace resolved types in lexicographic schema sort
• EventEmitter helper class has been replaces by SimplePubSub
• print_ast now breaks arguments over multiple lines

Special thanks to @cancan101, @mlorenzana, @wuyuanyi135 for contributing.