First beta release of GraphQL-core 3.1, based on GraphQL.js v15.0.0rc1.
In addition to incorporating the latest changes from GraphQL.js (see the list of changes in GraphQL.js [v15.0.0a1](https://github.com/graphql/graphql-js/releases/tag/v15.0.0-alpha.1), [v15.0.0a2](https://github.com/graphql/graphql-js/releases/tag/v15.0.0-alpha.2) and [v15.0.0rc1](https://github.com/graphql/graphql-js/releases/tag/v15.0.0-rc.1)), this release also contains the following improvements and changes:
* Change: Use `Undefined` instead of `INVALID` which is now deprecated (77)
* Use explicit `Optional` type on arguments (76)
* Remove unnecessary result type annotation from `__init__` methods (python/mypy5677)
* Better docstring for `ast_from_value`
* Better docstring for `lexicographic_sort_schema` (75)
* Fixed a minor issue when coverting lists in `ast_from_value()`
* Fixed a minor issue with `MapAsyncIterator.athrow()`
* Improved test coverage (trying to reach 100% for final release)
Contributors to this release were:
Cito, hoefling
Thanks to everybody who contributed by sending bug reports or pull requests. Please continue to report any problems you find in this beta release, so that these can be fixed in the final v3.1 release.