Add compatibility against Pytest 5.4+

[stanislavlevin]

The direct construction of nodes is deprecated since Pytest 5.4 [0]:

The construction of nodes now should use the named constructor
from_parent. This limitation in api surface intends to enable
better/simpler refactoring of the collection tree.
This means that instead of MyItem(name="foo", parent=collector, obj=42)
one now has to invoke MyItem.from_parent(collector, name="foo").

[0]: https://docs.pytest.org/en/stable/deprecations.html#node-construction-changed-to-node-from-parent

Fixes: #407

[coveralls]

Coverage decreased (-0.04%) to 66.821% when pulling 5d88081 on stanislavlevin:fix_pytest6 into 36e8452 on Kronuz:master.

[e0ne]

Thanks for the fix, @stanislavlevin