Use contextvars for the nursery fixture

[touilleMan]

Right now the nursery fixture is implemented in a pretty hacky way by patching the item object given by pytest:

pytest-trio/pytest_trio/plugin.py

Lines 282 to 284 in a972dc6

	@pytest.fixture
	async def nursery(request):
	return request.node._trio_nursery

I wonder if it wouldn't be better to use the contextvars feature now that it is availble (even in older version of python thanks to Yury's backport https://github.com/MagicStack/contextvars)

[njsmith]

No longer relevant after #50 made the nursery fixture unique for each fixture/test that requests it.