v3.2.0rc4

Pre-release GraphQL-core v3.2.0rc4, based on GraphQL.js v16.0.1.

In addition to porting the recent changes in GraphQL.js,
this pre-release also contains the following improvements:

• Use typed dictionaries for introspection results (#99)
• Deprecate FrozenDict/List, use tuples as node lists (#112)
• Experimental support for async iterables as list values (#123)
• Make print_block_string work with string proxy objects (#153)

Note that the use of tuples may break code that tries to modify the AST.

Please test this pre-release and report any issues, the final release is imminent.