pytest 5.4.0 deprecated `pytest.Function()` construction

[mjpieters]

See the 5.4.0 list of deprecations:

#5975: Deprecate using direct constructors for Nodes.

Instead they are new constructed via Node.from_parent.

and pytest-dev/pytest#5975.

This results in a failure to collect tests, see this example CI run:

____________________ ERROR collecting tests/test_signals.py ____________________
/opt/hostedtoolcache/Python/3.8.2/x64/lib/python3.8/site-packages/pluggy/hooks.py:286: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
/opt/hostedtoolcache/Python/3.8.2/x64/lib/python3.8/site-packages/pluggy/manager.py:93: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
/opt/hostedtoolcache/Python/3.8.2/x64/lib/python3.8/site-packages/pluggy/manager.py:84: in <lambda>
    self._inner_hookexec = lambda hook, methods, kwargs: hook.multicall(
/opt/hostedtoolcache/Python/3.8.2/x64/lib/python3.8/site-packages/_pytest/python.py:216: in pytest_pycollect_makeitem
    res = outcome.get_result()
/opt/hostedtoolcache/Python/3.8.2/x64/lib/python3.8/site-packages/pytest_asyncio/plugin.py:39: in pytest_pycollect_makeitem
    item = pytest.Function(name, parent=collector)
/opt/hostedtoolcache/Python/3.8.2/x64/lib/python3.8/site-packages/_pytest/nodes.py:83: in __call__
    warnings.warn(NODE_USE_FROM_PARENT.format(name=self.__name__), stacklevel=2)
E   pytest.PytestDeprecationWarning: direct construction of Function has been deprecated, please use Function.from_parent