replace gen.coroutine usage with `async def`

[graingert]

most functions that use @gen.coroutine were upgraded in #3706 #3242 #2934 and #2871 however there's still a few left

notably

distributed/distributed/comm/tcp.py

Lines 330 to 334 in f4c52e9

	@gen.coroutine
	def close(self):
	# We use gen.coroutine here rather than async def to avoid errors like
	# Task was destroyed but it is pending!
	# Triggered by distributed.deploy.tests.test_local::test_silent_startup

this Task was destroyed but it is pending! warning message is due to loop.stop() being called before all the tasks are finished. When using asyncio.run all tasks are cancelled and the loop only stops when all the tasks are finished and so this error should no longer happen.

see also #3245

Soft blocked by: Usage of asyncio.run

[fjetter]

Blocked by #6163