pytest: Tests: Derive from `pytest.Function` rather than `Item`
pytest: Tests: Derive from `pytest.Function` rather than `Item`

This way VSCode will still include us in the list of results.

See also: [vscode-python#8242] where it broke, [vscode-python#8245] which
is the issue tracking fixing it and [_get_item_kind] which is the function
which figures out whether this is an pytest test function or not.

[vscode-python#8242]: https://github.com/microsoft/vscode-python/pull/8242
[vscode-python#8245]: https://github.com/microsoft/vscode-python/issues/8425
[_get_item_kind]: https://github.com/microsoft/vscode-python/blob/411d1d4ae7e3884a8ef9294a0aa1688dc05d0f37/pythonFiles/testing_tools/adapter/pytest/_pytest_item.py#L530-L540

stb-tester/stbt-rig