asyncio.coroutine has been deprecated and will be removed in Python 3.10

[Karthikeyan Singaravelan]

Describe the bug
asyncio.coroutine has been deprecated and will be removed in Python 3.10. Using async def is recommended. Upstream issue : https://bugs.python.org/issue36921

To Reproduce

tests/web/test_websocket_server.py
55:@asyncio.coroutine
113:@asyncio.coroutine
138:@asyncio.coroutine

bigchaindb/web/websocket_server.py
99:    @asyncio.coroutine
121:@asyncio.coroutine

Expected behavior

Use async def syntax

Desktop (please complete the following information):

• Distribution: [e.g. Ubuntu 18.04]
• Bigchaindb version: master branch
• Tendermint version:
• Mongodb version:
• Python full version: 3.8

[davie0]

Thanks for reporting @tirkarthi!
Fixed in #2710