v0.7.1: Update `langchain` integration; fix EL task, model endpoints

🔴 Bug fixes

• Remove misguided check for fixed endpoints in OpenAI REST implementation (#429)
• Fix bug in EL task if non-highlighted entities are in text (#375)
• Update langchain integration to support langchain >= 0.1, < 0.2 (#433)

⚠️ Backwards incompatibilities

No backwards incompatibilities w.r.t. v0.7.0.

📖 Documentation and examples

• Link task mentions in readme to docs (#421)

👥 Contributors

@honnibal, @ines, @magdaaniol, @rmitsch, @svlandeg