Documentation: add Python client for etcd v3
- [txaio-etcd](https://github.com/crossbario/txaio-etcd) - Asynchronous etcd v3-only client library for Twisted (today) and asyncio (future)
- [dims/etcd3-gateway](https://github.com/dims/etcd3-gateway) - etcd v3 API library using the HTTP grpc gateway
- [aioetcd3](https://github.com/gaopeiliang/aioetcd3) - (Python 3.6+) etcd v3 API for asyncio
+- [Revolution1/etcd3-py](https://github.com/Revolution1/etcd3-py) - (python2.7 and python3.5+) Python client for etcd v3, using gRPC-JSON-Gateway
**Node libraries**
