Make use of TypedDict

[Cito]

We could make use of TypedDict in several places, for instance as return value of to_kwargs and introspection_from_schema.

Unfortunately, this needs Python 3.8, but we are currently supporting Python 3.6. However, it should be possible to add typing_extensions as additional dependency for Python 3.6 and 3.7 to support TypedDict for these Python versions as well.

[KingDarBoja]

Started to use TypedDict on graphql-server-core, those are awesome to work with and the typing_extensions does the job to bring backward compatibility to Python 3.6 and 3.7. Only thing that you must be aware is that you must provide all values to any TypeDict declaration as those doesn't support default values on class definition as far as I know.

[Cito]

This has now been implemented for introspection_from_schema in 24506f3.

Still needs to be implemented for to_kwargs.

[Cito]

This is now also implemented for to_kwargs, so we can close this issue.