Deprecation warning with pytest 5.4.0

[pllim]

Saw this in astropy CI.

==================================== ERRORS ====================================
________________________ ERROR collecting test session _________________________
../../.tox/py36-test-alldeps/lib/python3.6/site-packages/pluggy/hooks.py:286: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
../../.tox/py36-test-alldeps/lib/python3.6/site-packages/pluggy/manager.py:93: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
../../.tox/py36-test-alldeps/lib/python3.6/site-packages/pluggy/manager.py:87: in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
../../.tox/py36-test-alldeps/lib/python3.6/site-packages/pytest_doctestplus/plugin.py:456: in pytest_collect_file
    return self._doctest_module_item_cls(path, parent)
../../.tox/py36-test-alldeps/lib/python3.6/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 DocTestModulePlus has been deprecated, please use DocTestModulePlus.from_parent
=========================== short test summary info ============================
ERROR ../.. - pytest.PytestDeprecationWarning: direct construction of DocTest...
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 1.07s ===============================

[pllim]

Fixed by #103