Fix typesystem version to avoid errors

There's a new release of typesystem and SchemaDefinitions is renamed to Definitions which breaks apistar. apistar doesn't fix the upper bound of the typesystem version so it broke it.

To fix add typesystem==0.2.5 to the requirements.txt

https://teklia.slack.com/archives/CKUF04C3D/p1631704518001900