v8.1.7: Updated layers and extended requirements

✨ New features and improvements

• Add with_flatten.v2 layer with symmetric input/output types (#821).
• Extend to typing_extensions v4.4.x for Python 3.6 and 3.7 (#833).

📖 Documentation

• Update Gatsby for thinc.ai (#827).

👥 Contributors

@adrianeboyd, @albertvillanova, @danieldk, @essenmitsosse, @honnibal, @ines, @shadchin, @shadeMe, @svlandeg