v3.1.0b0

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, v15.0.0a2 and v15.0.0rc1), 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/mypy#5677)
• 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.